Firstly appreciate you using replays
In my opinion I feel tera shouldn't be banned because then there is no unique mechanic or trump card left to people's teams. The surprise turn in battles can be influential leading to tera types being a vital component in teambuilding. The fact that it's only allowed once per match is also quite important towards my stance on this decision. Not just that but with terrastralisation removed a lot of pokemon will find themselves to not be very versatile as they no longer receive the boosted stab they got from tera leading to the metagame being changed a bit too much.
NDBH is one of the most diverse metas available, you can make so many interesting sets that are viable and fun to use. They might not be the best options or the best mons but they can easily still perform well. Look at anaconja's teams for example. There are so many options that you can easily still have that "unique mechanic or trump card" on teams. And do you really think that surprise turn in battles is a healthy aspect? This has been addressed before but this possibility of a "surprise turn" encourages players to play passively. Players will avoid making aggressive plays with their breakers, constantly pivoting in defensive mons or Imposter. Your last point also makes little sense, are you implying that theres a lot of teraable mons in this meta that were not viable in G8 NDBH when there was no tera? Most mons that utilize STAB Tera are still going to be incredible in the meta (Zac-C, bugged PDon, Arceus-Ghost, Miraidon, etc) while the ones that suffer more will still have a definite niche, such as Flutter Mane.
Turn 12 where i terrastralised my zygarde into a water type to eat the ice beam is a the part i want to emphasise. An extremely unfair play for the opponent that actually changed the flow of the game. Pokemon like zygarde that have gained popularity in the current meta will lose viability and versatility if tera is removed and personally i feel like zygarde is like one of the best defensive pokemon in the meta.
This replay highlights exactly why Tera is problematic. With one click of a button Zyg reverses an matchup and immediately eliminates the opposing threat through the aforementioned trapping. Moreover, the opponent has even scouted the set and knew it was Ice Scales Zyg, yet it is still impossible to determine that the Tera-type is Water, because it can easily be something like Fairy, Dark, or Poison and would enable Kyogre to win that matchup. You even said it yourself, its an extremely unfair play for the opponent. Why should we keep this "extremely unfair play" enabling mechanic in the meta?
Regarding Zyg, you are absolutely correct. Zyg is absolutely the best defensive pokemon in the meta, and Zyg will lose viability and versality if Tera is removed. However, that does not at all mean that Zyg's viability plummets. If you check the previous gen thread, you will notice that Zyg is ranked solidly in A, and this is in a meta without Tera, with an unnerfed Glacial Lance (Zac predominantly uses Magical Torque which is far far less threatening, FC Zyg eats 2 Band SoR Torques which is hilarious), threats like Ash Greninja that Zyg is not the best at handling, as well as an absence of stuff like Covert Cloak (Zyg is a good user of it and is strong against it since Thousand Waves bypasses it) and Ability Shield (Zyg good user). I can assure you that Zyg will still remain as likely the best defensive pokemon in the meta, being immensely splashable and blanket checking/midgrounding so many threats.
In addition, I would personally argue that with Tera allowed, Zyg is very borderline broken and I don't think its a stretch to potentially see tiering action on it if Tera stays. Stuff like Water or Fairy type Zyg bolstered by FC, AV, or Scales is absurdly difficult to force out or deal with at all. It is basically an auto-include on every team since it can perform any role and it can be argued that you are at a disadvantage for not using it. Zyg is so good at its role that its eclipsing the majority of other defensive mons. Zyg completely warps the offensive meta around it. Everything already basically needed a strong super effective move against it and now need to consider bringing coverage for the common Tera-types it runs. Yet types like Water and Fairy have such few weaknesses, and in the case of Fairy very poor moves to use.
In this match tera was completely neglected, he used his tera for a small play with his chansey that got rid of my deoxys that didnt really threaten him much. In this battle you can see that I didn't at all terrastralise any pokemon because it wasn't needed at the time, it shows how terrastralisation isn't as unbalanced as presented as it only provides a defensive advantage in the current metagame. This replay proves that the shift in advantage is only situational leading to my conclusion
I don't know about "didn't really threaten much" when your Deoxys 2HKOs at worse their entire team with prediction. Also it can be claimed that the defensive potential of Tera is more uncompetitive (not necessarily stronger) because its actually a lot easier to play against STAB Tera since you can just always consider a boosted damage output while with defensive Tera the free turn it generates can be immense.
For example, consider say Specs Miraidon, a player can always prepare for the Tera Electric or Tera Dragon power by using something like Scales Dialga-O + hazard/chip pressure to weaken Dragon Energy + a Fairy-type like Zacian-C. However say against a PH Arceus-Fairy with known Fairy/Fire coverage and Tera Fire. The player cannot utilize their offensive Steel-move Zacian-C to offensively check this since it can Tera Fire anytime, and defensive checking is also more difficult since Tera Fire also provides a useful power boost on top of defensive utility.
The metagame also revolves around many good mons with bad typing terrastralising into something better.
But how this is more balanced? Isn't it better to have high bst mons be balanced out by less ideal typings (such as Zyg) and lower bst mons but with strong typings (such as Mega Swampert). Mega Mewtwo Y is in fact a perfect example, if MMY could Tera it would be absolutely busted, since it would be able to get rid of its very poor typing while retaining its absurd stats.
In this battle you see at turn 9 both players trade crits and my chansey dies. Over there I actually had tera ghost on the chansey and could've easily clicked the tera option to eat the hit and risk a speed tie. But instead I misplayed and chose to not tera to keep my extra stab to try and KO the opponent. Because for all i knew there was a great chance of him also clicking oblivion wing hoping for the recovery to benefit him more than me. This goes to show that you need to properly read to click the tera button. You dont just click the button and flip the matchup instantly.
But how is this a "misplay" and how is this showing that you need to properly read to use Tera. If opponent clicked Tera-Flying immediately the neutral OWing does the same damage as super effective Thunder Cage, and regardless without the crit the 1v1 is determined by speed ties (even with crit if you moved 2nd first turn then 1st second turn you win). This replay doesn't really show anything since you didn't bring any Triage checks with Lucky Punch Imposter being your best check.
Well the trick is that these good pokemon that could make great potential of terrastralisation aren't allowed to terrastralise thus leading to not too many shifts in the situation.
The thing is these aren't very good pokemon. The ability to Tera on defensive mons is such a massive benefit. One can see this being reflected through the kickoff tour stats. Dondozo went 5-0 while Mega Slowbro went 6-9. Yet Mega Slowbro sports more physical bulk (on a physical wall) and far less passivity. Dondozo being able to Tera gives it so much more flexibility to soft check stuff or emergency cripple a threat. While the VRs aren't posted yet you can tell that the inability to tera is a massive detriment (see Deoxys-A's soaring viability compared to last gen, and all the defensive Megas being worse).
One last point too, proving that the shift in advantage is only situational. If he hadn't gone tera flying at that point how much would the game change? he had that guaranteed 2HKO on both defensive mons and guaranteed OHKO on anything that could threaten the yveltal. Thus allowing me to repeat that terrastralisation doesn't just cleanly flip matchups questioning why it's being suspected again.
Again this was a specific game where that Yveltal had a naturally incredible matchup. But imagine a better matchup for you where you had something like non-Band Zacian-C that would check it. With Tera-Flying though Yveltal can 2HKO Zacian-C while the only move that Zacian-C can potentially OHKO back after Life Orb recoil is Pixilate Population Bomb.