I'd personally go around the propositions in this order:
1. Showing Tera type at Team Preview:
This is, I'd say, the best way to ever implement a nerf on Tera. It wouldn't be a complex ban, but simply a Clause, which makes the process easier, and it's way more efficient at dealing with the Tera problem than the other methods, bare maybe the next one. For me, the idea of Tera-types being visible in Team Preview seems like the best thing to implement. We already have rules which imply that player would reset a match if double sleep was attained (Sleep Clause), and specific banlists which imply they agreed on a ruleset before the match (Ubers, BLs, etc). So clearly, implying that each individual would share the Tera-types of their Pokémon before a battle (during the team preview) doesn't seem that big of a stretch, just a gentleman rule like all the other Smogon clauses.
2. Banning Tera Blast:
It mainly limits the bullshit, but may not fix the real problem. Although I do think that Blast should always be suspected/banned before any action is taken on Tera itself. Deleting Blast could seem like the right call, as it only makes Tera sweeper with actually good coverage threatening, making it an ‘easy’ “we ban this Mon because he’s to good”, but the versatility and unpredictability aspects are still there for way too many Mon.
3. Limiting the amount of Pokémon on any given team that have access to possibly Terastallize during a battle:
Seems like the “least bad” idea of the remaining three. Way too hard to implement, and a complex ban is always a pain in the ass to justify and assert into a ruleset. If it’s done well, maybe it could be a great idea, but I doubt it. It sound a bit to confuse and gimmicky to be a solid solution.
4. Limiting Tera typing to previously existing STAB types:
This may actually be worse than banning it completely. People will keep complaining about dual typed changing their type and clearing games, and it removes a lot of depth to this mechanic, basically transforming it into a ‘Super Z-Move/Gem For The Rest Of The Game” thing in a lot of battle scenarios. This is definitely the worst solution in my opinion, and I’d almost prefer the mechanic to be banned than to see it fall like this.
5. Outright ban:
This is a last resort, a button which, in my opinion, shouldn't be pushed unless we're certain it's the only option we have left. I'd say the mechanic should not be banned, as it's not nowhere near Dynamax. This doesn't justify not banning Tera, but it's certainly important to consider that we do have other alternatives, which should imo be prioritized.
Those are of course just my thoughts, I can see arguments being made for each of those options, and there's no right or wrong in there.
EDIT : I just saw the 'limiting to one, and showing it at preview'. Best of both worlds.