Hi, I played this ladder a lot already around 600 times, and have gotten peak number 1 multiple times, most recent being 1950.
I generally think for now, tera is balanced and adds an extra layer of prediction and mindgames which can create a very fun and competitive metagame, and would vote on
at least not after one or two major tournaments...
So far, terastalization has been used by pokemon to either get a big offensive boost, defensive boost, utility or even as a bait. Example of each being:
Offensive Boost: Chien-Pao terrastalizing into dark type to get a harder hit on walls like dondozo or alomomola, Roaring Moon Terastalyzing into flying to beat mons like great tusk, Dragonite or Lucario terastalizing into normal for an explosive extreme speed
Defensive Boost: Skeledirge terastastalizing into a fairy type to get better defensive match up againt its weakness like dark and ghost, or Hatterene terastalizing into fire type to nullify its steel weakness and hit them back with stab mystical fire
Utility Boost: Garchomp Terastalizing into a ghost type to prevent rapid spin attempts from the donphans or cyclizar
Bait: Glimmora Terastalizing into Grass to beat Great Tusk lead or Chi-yu Terastalizing into flying and having the move taunt to shut down Ting-Lu
It adds a layer of complexity to every match where every turn you need to take into account "hmm will they terastalize this? if so, what will it be?" in a way that is balanced and fun. For example, if you have breloom and the opponent sends out kingambit, well you just predict the tera ghost and go for spore. Or if you earthquaked a glimmora on its spikes, do you spin now and clear hazard + kill? or do you earthquake predicting the tera ghost. You need to play it intellegently to get the most out of it, both in the builder and battle. Do you tera now to block the spin? or safe it for later to get a sweep?.
This new mechanic that gives the ability to change type on command, or adding an extra oomph to a hit adds so many ways to build a team or play using it or againts it. The outplay potentials of predicting with tera is massive with every turn having a chance of something suddenly changing types, and then you successfully predicting that. The potential in the builder is also great with having some pokemon to be a lure for something else, like garchomp baiting in chien-pao by terastalizing into fire type, resisting icicle crash and hitting back with fire blast.
Its also not impossible to predict, and you usually can tell what pokemon might tera into what based on context of teams. If you see lead glimmora or masquerain, its probably ghost. If its a tank like skeledirge and garganacl, you can safely assume a type that has a lot of resist like water or fairy, if you see kingambit going in vs your chien-pao or great tusk, its probably flying or ghost.
Terastalizing also doesn't just win you on the spot, its not like dynamax where when you click it, you just win with double hp + super boost, no, you only change type or gain an adaptability bonus on one type of move, there is also a cost to it. Yes getting stab E-Speed on Dragonite is powerful but you don't just win on the spot after you do that, you lose your ground immunity and resist to water, because of that you are now vulneable to pokemon like great tusk and body press corviknight which you beat before.
It's not inherently broken because there are a lot of counterplays to it that is reasonable, you dont have to specifically prep or predict for tera, but if you do you will be rewarded for it. In that same regard, Terastalization is also not inherently uncompetitive since using it properly and to its upmost potential requires positioning, playing around and general metagame knowledge. If you know whats popular right now, and what counters it, you have a greater advantage with building using tera.
Here are some replays i have/found where tera is used in ways to predict the opponent's move/attack/pokemon.
https://replay.pokemonshowdown.com/gen9ou-1714538341-sjr5ag6o464nhx3jryyt0ouqqwpjsdppw tera poison baxcalibur to beat hawlucha's close combat
https://replay.pokemonshowdown.com/gen9ou-1723868095-0xmveu8qiie731lv9ygj4ads6hagyujpw Garchomp terastalizing into fire type againts my chien-pao predicting icicle crash and hitting it back hard
https://replay.pokemonshowdown.com/gen9ou-1723887972-0hcvbh5ew75x7s5i4arxnmb2hye5kbepw luring my opponent's great tusk with lead glimmora and then nuking it with energy ball
https://replay.pokemonshowdown.com/gen9ou-1720630700-mpwuirnzyev1hqinbj8q22h70uo8qvipw Garchomp in midgame terastalizing into ghost type to stop a rapid spin
https://replay.pokemonshowdown.com/gen9ou-1723911485-pcvy5vqveqeuso4uf73x6tz2ok6g706pw terastalizing my glimmora to grass type to beat my opponent's ting-lu, only to then having my opponent terastalizing their ting-lu into a poison type to counter my grass glimmora and absorb its tspikes.
i do apologize for only having mostly Lure/Utility tera-showcase replays, since i have a habbit of not saving replays, but do assure it can be used defensively and offensively.
If the council wants to do someting as of now, they should try by suspect testing the broken pokemon first instead of terastalization as the metagame hasnt been out for too long and the potential of building and playing with it hasnt been fully realized yet as there have been no major tournaments for good players and builders to discover its true potential.
Terastalization after all, is the new generation's gimmick and banning it or testing it before a major tournament after showing that its not broken or uncompetitive doesn't seem fair.
TL;DR
Do not take actions againts tera, at least not before having one or two major tournaments as it is a complex new mechanic that brings a lot of nuance to the metagame that can be used in many different ways and adds an extra layer of prediction and mindgames both ingame and in the builder. It is not unbalanced because using it properly takes skill and it doesn't just outright win you the game most of the time. It is also not uncompetitive as it does not include major luck based elements.