With
Finchinator saying "get creative", how about this. It's asking for a lot but I am proposing a combination of 3/4 restrictions together.
I think having each Pokémon's individual tera typing's having their own ban list (example: chi yu is unable to tera dark, or tera fire) IN ADDITION to the restriction of tera's shown on team preview, AND AFTER TEAM PREVIEW BEFORE THE MATCH STARTS you pick which Pokémon you will tera, is the best possible outcome.
This can help manage the balancing aspect of it or the "swingyness" or "power" of the mechanic many people complain about by getting rid of the ones that pressure teams too heavily like chi-yu tera fire.
The predictability element of tera on team preview can help alleviate the guesswork which is very helpful for newer players trying the tier since it'll feel less overwhelming allowing for a thriving player base.
And finally, being able to pick the tera typing after you see the opponents tera typing's off team preview, limits the potential for 50/50's while allowing for the ability to micro manage bad MU'S still, while also having a skill element that rewards better players for predicting which of the opponents Pokémon they will decide to tera, allowing for a more in depth meta.
If anything, tera typing's when chosen after team preview can automatically be said, if the unpredictability element remains too strong even with the 3 former restrictions. What I mean by this is, after team preview lets say there is 40 seconds to choose tera typing. In the chat it would say "opponent is using tera X on X" and then the game starts. This could alleviate the problem of tera being too "unpredictable" if it proves too much the restriction of team preview tera types and picking tera on 1 Pokémon after team preview. At that point it becomes a battle of "ok when are they gonna use their tera water on clodsire" instead of trying to figure out which tera the opponent chose to begin with.
Obviously having a sub ban list for a mons tera typing is asking a lot from the OU council, and these 3/4 restrictions together doesn't solve everyone's qualms but I believe this can limit 50/50's, still allow for a similar scale of depth in comparison of how the mechanic is in base game, MU fishing is still largely diminished, and much more.
we could also go without sub bans as well, and just ban the broken mons with tera which are like 2 (ape/chi yu) but even without tera they are dumb anyway so, but I propose sub bans as a "last resort" to the haters.