gonna go with
ghost/fairy
Weaknesses Ghost, Steel
Resistances Bug
Immunities Normal, Fighting, Dragon
Super-effective Against Ghost hits: Ghost, Psychic. Fairy hits: Fighting, Dragon, Dark
Ineffective Against Ghost is resisted by: Dark, Normal (immune). Fairy is resisted by: Fire, Poison, Steel
Strengths
Flexible, powerful STAB options Both Ghost and Fairy are extremely potent STAB types, to the point where it's possible for CAP 23 to only run one of them on a set and have a extra move slot to work with, which ties back to the former desire to allow CAP 23 flexibility. It also provides STAB on Spirit Shackle, sorely needed due to the lack of things it hits for SE and its 80 BP, allowing us to fulfill the concept.
Powerful Immunities There are a lot of fighting moves flying around in the cap metagame, so swapping into those for free is super cool. Dragon is a bit rarer, but there are still
some out there.
Not weak to pivots We resist U-turn and aren't weak to the STAB moves of most pivots in the CAP meta, allowing us to face them down without fear of getting struck down.
Few weaknesses over-all We're only weak to other Ghosts (somewhat problematic), and Steel (not really problematic at all).
Dunks Ghosts Ghosts are immune to being trapped, so being able to smack them down (even if we can't safely switch in on them) is nice.
Dunks Tomohawk Tomohawk is prime jerk #1 whom stops all set-up sweepers in their tracks. If we want to be able to explore set-up space with trapping, it's highly ideal that we have the ability to dunk Tomohawk even if he can stop a set-up. Fairy STAB accomplishes this by dunking Tomohawk with 2.5x power moves.
Not weak to Stealth Rock While resisting Stealth Rock would be ideal, taking only 1x damage from it is vastly preferable to taking 2x or even 4x damage from it.
Weaknesses
Few Resistances A lack of resistances means that CAP 23 will be worn down throughout the match, being able to only swap in for free on a few (albeit common) types and will have to compensate for that fact elsewhere.
Weak to Celesteela Sadly we're probably going to lose to Celesteela with this type almost regardless of our stats or movepool since Heavy Slam is a 120 BP 2.5x damage dunk and will almost certainly OHKO CAP 23 or come close, even with Celesteela's relative lack of atk and low to non investment. That being said, it's kind of a fool's errand to beat
every pivot/bulky pokemon, and I believe it's acceptable to lose to one whom gets worn down throughout the course of the match anyway due to only having leech seed for recovery.
as we currently have effectively no weight and thus a default of 0, i am assuming heavy slam will hit for 120 bp
Why This Typing is Pro-Concept
To sum up everything that's pro-concept here and not just strengths of the typing: We have a set of two STAB options which both can deal with many bulkier pokemon in the meta by virtue of not running into many, if any resistances between them. Both of these are potent enough on their own right that CAP can run a set without one or the other, which means that while STAB Spirit Shackle is a move CAP 23 is incentive to run, it does not
have to run it and can exploit that fact by running only Fairy STAB on some sets. CAP 23 is not weak to volt switch, pursuit, or u-turn with this typing, and in fact resists U-turn. CAP 23 is also not weak to the vast majority of pivots, and is not walled by them. The fact that Fairy dunks Tomohawk allows us the possibility to explore set-up moves, which naturally compliment both trapping and the fear of being trapped. Finally, while this typing is weak to ghosts, it also dunks them, which is a bit of a double edged sword but does mean we don't run into the situation where ghosts can switch in/out for free.