In your opinion, what Pokémon are broken with Tera and would they be broken without it?
Every priority mon that's at least half decent. Chien-Pao, Kingambit, Scizor, Breloom, Dragonite, Talonflame, Azumarill, Basculin, Quaquaval, and even Lokix can all bring in stupid damage under the right circumstances at +1 or +2. I'm sure there are others I've momentarily forgotten.
The priority counters are also largely not so good right now. Farigiraf, Bruxish, and Tsareena aren't good. Psychic Terrain is very rare.
When do you think is the best time to use a Tera?
Whenever you can get multiple kills with it. Or if you can get a single kill with a Tera'd priority user that could be useful later in the game. Outside of that, it's very rare that you can get merely a single KO with a surprise Tera and that's actually enough to win you the game, especially if you Tera'd first. It sounds insane, but the Meta is just so hyper offensive that one kill usually isn't enough to justify a Tera.
How does a Pokemon’s checks/counters change when it Teras, and how significant is this?
It's very significant when a pokemon changes its type, and a move that was supposed to be super effective against it and KO it is no longer super effective. But the biggest problem comes after that with offensive retaliation. That's the thing that can bring you from being in a winning position and playing perfectly to losing the game in a single turn with little to no chance of recovery.
How does Tera force 50/50s in a 6v6 metagame?
By making you predict not only when the opposing pokemon will Tera, but which Tera type it will be. I'll give an example of Roaring Moon. Roaring Moon can be beaten with Breloom's Mach Punch, unless it has Tera Flying. In which case a Bullet Punch from Scizor would be the better answer. But it might not be Tera Flying. It might be Tera Steel, or it might not Tera at all, in which case Breloom would be the better answer.
There is no way to know the correct choice here unless you know ahead of time what the Tera type of Roaring Moon is. This is why I have favorable opinions on the Tera team preview option. Because it brings the amount of factors you have to predict to something more reasonable. Roaring Moon still might not Tera. But at least you know what Tera it can change to so you can plan accordingly from the beginning of the match rather than finding out after your pokemon died and the sweep is incoming.
How much of an impact does Same Type Tera have on a game?
An absurd amount. To be blunt, the double STAB bonus is BS and really shouldn't exist. You can get everything right and still be just brute forced through by same type Tera Mons firing off powerful attacks like Chi-Yu and insane priority like Chien-Pao. Literally every priority user I listed except Dragonite abuses double STAB. There would be
far less mons that will need to be banned without this. It's easily the worst part of the mechanic.
For those of you looking at complex bans, how do these solve the problems, and which problems are you looking at (be specific and give examples).
I think the issue is two parts. I've said I like the Tera team preview option to help with the "50/50s" or whatever you want to call the insane amount of variables and the pressure that puts on everyone that results in having to guess too much.
The second issue is the double STAB for same type Tera. If there was a way to get rid of that, or at least stop same type Tera pokemon from interacting with or using bonus multiplier items, this is the way to go. But unless this issue is addressed, none of the attempts to compromise here instead of outright banning Tera would ever work.
So it has to be some combination of the Tera team preview and limiting the damage multipliers for same type Tera. The ladder is the harder but more essential part. I would love to see people talk more about this aspect.