I feel like you are complaining a bit too much about "you have to guess, not react". This is Pokkén, it is literally a guessing game, much like rock-paper-scissors, but with more options and variables of course. (But I see where you are coming from when you complain about things like Braixens light screen and Chandelures ability to break the attack triangle. You're not wrong.)
I main Suicune, also a bottom 5 character, and for one thing, I sure as heck know how to live by the guessing game, as that is the only way Suicune can win. IMO, Suicune is the character that needs to rely on RPSs the most, as he otherwise has bad options and is very punishable overall. He can't just throw out moves willy-nilly, or else he will be punished hard. He's not a great character, but I try to do the best with his tools anyway and focus on improving as a player.
Yeah, Gardevoir isn't great, and to be fair, much of what you wrote are warranted. But I feel like Gardevoir players just throws out moves and hopes for the best. It gets predictable at last, making her very easy to punish. But I don't have Gardevoir's perspective, so I really can't say more.
Now, I'm all for complaining about bad/underpowered characters, because no one will do it for you. There are still balance issues in Pokkén and underpowered characters like Gardevoir and Suicune need more love indeed. I honestly don't know what to say more.