If you've seen my previous posts then you know my stance, but I want to make a post about it anyways, if I can get reqs then I'm voting Ban.
I'm going to make an analogy to fighting games: Let's talk about neutral, advantage, and disadvantage states real quick. As the name implies, neutral occurs when neither player has an advantage and both are trying their best to get one with their moves, techniques, and stage control. Advantage and disadvantage happen when neutral ends because someone got a hit in, the player in advantage wants to push said advantage as much as possible, and the player in disadvantage is trying to reset the match to neutral, or very rarely, pull it into an advantage for him. Keep in mind that advantage doesn't always mean winning and disadvantage doesn't always mean losing, because these states can change very frequently.
Let's apply this principle to Pokémon. I feel that the only truly neutral moments are team preview and the first turn, after that the game wildly swings from semi-advantage to semi-disadvantage states because of Pokémon switching and predictions, until someone gets a correct read and makes a serious dent, gets a status effect on a key opponent, or KOs a mon, then the game truly enters the advantage-disadvantage stage. Items like the three Choice items and Life Orb increase the risk of your plays but also make it easier to get this advantage by increasing your speed or the power of your moves. Getting hazards in and Taunting/KOing the opposing Defogger/Spinner? Advantage state. Pokémon is surrounded by this very system, and every move you make, including switches or sacking a mon, affects the game via this idea.
However, Dynamax deletes this very delicate, ever-changing concept. By being able to break away from Choice locks, Encore, and avoiding phazing like Whirlwind, you can punish your opponent for what would normally be a correct play to deal with your play. Switching into a counter when something's choice-locked won't work either because now they can just Dynamax and choose a coverage move. You're essentially breaking the concept of advantage and disadvantage. Your opponent punished you for a wrong choice, and yet you're suddenly more powerful than before, your disadvantage state is basically gone for three turns. It's comparable to a character in a fighting game having a move that comes out instantly and knocks the opponent very far away from them, removing their disadvantage. Not only that, but suddenly all of their other moves have drastically increased power and a secondary effect that always benefits them for 40 seconds.
What I want to demonstrate with this analogy is, no matter if you make a right play, a lot of the time the opponent can essentially punish you for doing the right thing by Dynamaxing. This isn't even taking into consideration the most problematic Dynamax users mentioned in the OP, that can easily get in, for example, after you KO an opposing mon. They then proceed to snag the advantage you had, and make huge progress towards winning the game by forcing a switch, which is usually followed by a setup move, then Dynamaxing and KOing, on average, 2 mons. By the time Dynamax ends, they have acquired even more boosts and are harder to deal with. It's a barrage of incredibly buffed moves and stat increases that becomes increasingly harder to deal with as it goes by.
Another point that I want to make, separate to my analogy, is that I have yet to see any solid argument for keeping Dynamax in. Keeping it because "then we won't have a gen gimmick and that's boring", saying "it's only 3 turns", "we could just ban the best abusers" or "both players have it so it's balanced" are all terrible reasons to keep something like this in. I don't understand why we would do so many bans and even potential clauses just to keep the problematic mechanic, when banning it would instantly fix it all. Maybe I'm biased because I despise what Dynamax does to the competitive metagame, but I tried to be as unbiased as possible and scrolled through both threads filled with discussion on the subject and never did I say to myself "You know what? That's a good reason to keep Dynamax in". So far, this thread is filled with reasons to ban Dynamax, and all the Pro-Dynamax arguments have huge flaws or are just downright silly suggestions.