So the main issues with Tera that I've personally felt are as follows:
(As a note, I've been on the Pro-Tera side pretty much the entire generation, but I'm second guessing my previous opinions)
1) Each turn before Tera is used applies relevant pressure and false tempo
I think this specific part about Tera is what makes it such a feel bad situation. The issue with Tera is that you gain false tempo simply due to the threat of Tera potentially happening. Bear with me with a quick anecdote of a potential situation:
Your Breloom has successfully gotten a kill, and your opponent brings in Roaring Moon. The Tera flying is pretty obvious here since you will be outsped, and Tera flying will resist mach punch, so you'll get revenge killed by acrobatics.
But you realize, you don't actually know if your opponent will commit to this plan. Are they actually Tera Flying? Or are they bluffing it? What if they don't use their Tera and you switch out for nothing? While it is likely that your opponent is going to Tera, there is no guarantee that it will happen, and if don't you are put at a severe tempo disadvantage. I wouldn't call this a 50/50 necessarily, because it is more likely the Tera is going to happen than not, so it's more like a 80/20, but the issue is that there is any semblance of chance that you can lose severe tempo due to the threat of an unknown. In any logical situation, a Breloom would check a Dark/Dragon, but in this case you might not. This makes this situation specifically dangerous due to Tera. And will swing the course of the game dramatically.
This specific example is definitely an extreme one, since again it's reasonable to assume that you will see the Tera Flying, but the issue I'm attempting to bring up is that these small prediction based decisions are placed each turn of the game which can swing relevant decisions. You gain an active disadvantage if you use your Tera before your opponent as they still can hold the looming threat of switching their typing, or bringing an adaptability nuke (or both) in front of you. If it were only a one turn 80/20, like the example above, it'd be one thing, but you're having to deal with 90/10 and 80/20 based predictions the entirety of the game until the trigger is pulled. There is always the question of "will I lose a relevant 1v1 due to a tera?" that is present each and every turn in a game.
2) There is potential for multiple types per Pokemon which causes false tempo
Some mons have pretty defined Tera types that they use. Annihilape usually runs Water, Chien-Pao likes to run Dark, Valiant likes to run Fairy. However, this will not always be the case. Annihilape could be Fire, Chien-Pao could be Ice, Valiant could be running some off meta choice such as Ghost.
While the issue is not necessarily the powerlevel of these choices, it's (like up above) the "false tempo" that is gained from the threat of the unknown. How are you supposed to know that Annihilape is Water or Fire? Sure, most of the time it's going to be Water, but what if you try to Will-o-Wisp it and it turns out being Fire? You just lost a turn of tempo to no fault of your own (in fact likely making the correct play).
Some mons even run multiple types effectively. Chi-Yu is commonly seen with both of its typings and even Fairy or Grass, Roaring Moon can be Steel, Dark, or Flying, Espathra can be Fighting or Fairy. While there are more common choices, there is no guarantee that you are correct until the trigger is pulled. While you can oftentimes deduce which Tera the Pokemon is based upon moveset, much like scouting items, sometimes there is no realistic way to do so, such as if a set has not been revealed to you, or the set is the same regardless of the chosen Tera (such as Chi-Yu).
Now what do I like about Tera?...
Tera opens the meta to interesting new movesets, and extends the reach of Pokemon versatility
Tera as a mechanic allows for some incredible moments, and makes Pokemon such a skill expressive game when it comes to the team builder. You get some new Gen 9 specific toys such as Acrobatics Roaring Moon, EKiller Dnite, Fairy Unaware Wall Skeledirge, Bulky Fairy/Water Garganacl, Grass Iron Moth etc. All of these are very interesting applications of the mechanic which allow for Pokemon to truly be at their maximum potential, and allows for making your team more versatile in the builder. Also, due to Tera Blast specific Pokemon essentially gain access to Hidden Power if they so choose to opt into it.
I think this is integral to Generation 9, and is the part of Tera which I would love to see stay (if not all of it). I think removing this from the game is disingenuous, and unless dire, should be highly considered to stay.
So... what do?
Here is my proposed solution, albeit a bit complicated:
On Team Preview, each player selects which Pokemon they intend to Terastallize. This is done at team preview to allow for adjustments. As an example I could choose if I wanted to Tera my Skeledirge to Fairy, or I could choose to do my Roaring Moon to Flying. But I only get one.
After Both players select, the Pokemon which will be Terastallizing are revealed to the opponent, in addition to their type which they will be Terastallizing into.
Once this Pokemon is set into the game, if the Pokemon selects a move (so doesn't switch out) they MUST use their Terastallization. This prevents any Tera mindgames, and the Pokemon's type is always predictable to your opponent. There are no more 50/50 guessworks.
I think this 3 step process, while complicated, does a few things
1) Not showing ALL Tera types in the preview limits set scouting
2) Forcing Tera on the first available turn, in addition to knowing which type it will be, prevents guesswork from either side
3) We can keep novel movesets, and teambuilding is versatile
4) You can react to your opponent's team to make a more informed choice on what to Tera
I'd love to hear what others have to think about this, it is a bit complicated, but I think it keeps the spirit of allowing the expressiveness of Tera, while not sacrificing competitiveness through set scouting, or uncomfortable unwanted guesswork.