This was a pretty interesting core to build around and I ended up running through about 5 different builds with things like Auto/Curse Mega Steelix with screens support, which ended up being impossible to setup but could decimate if I was able to get a couple boosts off, to Z-Safeguard Golurk with PuP. Ultimately I ended up with sandless Ground that I tried to build around being able to manage Bug and Psychic like you asked, but also being a capable team against types like Fairy, Flying, and Water. Starting off, I decided to change up the sets for both Diggersby and Landorus quite a bit to really focus on what roles they should be performing. I turned your
set into a Spikes suicide lead version with a Focus Sash so that not only could you get up hazards and get a hit off, but you could also preserve it in case you would need the Sash to check threats in certain matchups such as with Mega Sharpedo or DD Magearna. I ended up giving it dual STAB with a priority option in Quick Attack in case you can't outpace a threat after Diggersby's Sash is broken. I also generally preferred using Earthquake over Knock Off so that Diggersby didn't just turn into setup bait against resistant Mega's and Z Move users, although Knock can help to check Choice users and crippling defensive cores, such as Normal's Evio core. Frustration is mandatory to avoid allowing relatively free setup by Pokemon that could 6-0 your team such as Mega Gyarados. With your
set I chose to change it slightly after adding Nidoking and Seismitoad into the mix. I changed Sludge Wave to Rock Slide in order to provide a solid check to Bug- and Flying-types, namely Volcarona and Mega Charizard Y, while also giving Landorus pseudo-EdgeQuake coverage. I decided to change Substitute to Hidden Power Ice in order to catch opposing teams off guard, especially with regards to threats that could really punch some serious holes into this team through raw power, such as DD Landorus-T, Z-Fly Salamence, and even an opposing SD Garchomp. This also gives you a Pokemon that can pretty much dismantle Flying and Ground teams late-game, especially after a Rock Polish. Since Landorus is now carrying physical coverage as well, I gave it a Hasty Nature as well, providing relatively even bulk in both defenses. As for the last four members of the team, it basically came down to what the team needed, namely assured speed control, a wallbreaker to pair with Landorus, a Water check, and a setup sweeper/Z-move user. The first of these being
, which with Choice Scarf and Steel STAB can help to alleviate the overbearing threat that Fairy's Choice Scarf Tapu Bulu carries within the metagame as well as putting out fairly reliable damage thanks to Mold Breaker removing any Ground immune shenanigans. It's final move is Rapid spin over additional coverage namely for aiding in preserving Diggersby's Focus Sash as well as removing Sticky Web which can easily sway the team's matchup against Bug well into the latter's favor. Next up on the team I chose to go with
, but carrying only Poison STAB in order to maximize its breaking potential for bothersome defensive cores for Ground teams, namely Flying. This set does come with some risks however, as it is now much more difficult to break Steel's defensive core if your opponent still has a healthy Heatran. Since Nido is cursed with 4MSS and because the types this team would struggle with the most, i.e. Flying, Water, Grass, can be shredded by this set, I felt that running BoltBeam coverage was simply too good to pass up on. I also gave Nidoking 27 IVs in Defense in order to make it 1 point lower than its SpD, protecting it from being Download bait for Porygon-Z, which could potentially 6-0. After Nido, I decided to bring in the team's Water check through the form of
, helping to keep the momentum for this already offensive team and providing some much needed utility options in Stealth Rock and Toxic as well. I went with Sludge Bomb over Sludge Wave to give a greater chance for poisoning a target while giving up a meager 5 power. Scald was added for Seismitaod's best Water STAB and for fishing for important Scald burns as well. Lastly, I decided to go with Groundium Z
to round out the team and provide the team with a solid setup sweeper. With this set, you have the option of using a Tectonic Rage for a 175 BP nuke.