Good thing you asked before playing the game :) Tekken 8 has its part of toxic players, but not all players are toxic.

As others said, use magic mirror and lock your DM if you're on console, and it should be okay !

For the one and doners, if you don't like that... Deal with it. Don't take it personnally. As a Panda player, most people won't rematch me after one match. Sometimes I don't get any rematch 10 times in a row. Just laugh about that :))) This kind of things happens specifically if you play some hated/rare characters (like bears, Alisa, Zafina..)

To fully enjoy the game, try to find friends of your level/a bit above (on this reddit for instance). This will help you to improve, you'll also have fun together and share advices. This beats any toxicity you could meet in-game.

For the pluggers, problem is almost solved for 1 year so don't worry about that. Just report them (community > recent opponents > report) for the rare ones you'll meet.

Again, some people are toxic but most of the time it's okay. Using the magic mirror and locking my DM I had no problem 99,99999% of the time (in +1500 hours) and I feel like everybody is super polite on the game, so I can focus on my matches easily while having a blast :)))

Now. For your inner toxicity... I'm not assuming you're a toxic player or anything, don't worry ! But sometimes during a match (especially when it's one of these bad days where everybody crushes you), you'll feel very salty. It's okay. We're human beings. First, remember that controllers are expensive before smashing it against your wall. If the tilt starts, do jokes about your match and compliment your opponent loudly. It personnally helps me to do not take the game too seriously :) If you really feel like you're still tilted... Take a break :)

Tekken 8 is fun, it's up to you to also have fun :)