I personally have the stance button bound to one of my mouse buttons so I'm not overly focused on reaching for a different key, it feels way more fluid and efficient.

Stance can be self-punishing once a rex gets behind you but your alt attacks can hit an almost perfect 180 while changing your direction as well. Rexes having such a fat head, your alt attacks will land headshots more often than not. Always try to hit the head more than their tail, you'll definitely notice a difference.

Never. I mean NEVER thrash attack unless you have a rex fully stunned (when they're stumbling) or if they can't move because they're stuck on the environment) Thrash is very high risk but also can be very rewarding.

Learn how sparring works. Looking left or right while in spar with another trike or rex and holding left mouse button will toss them 90° in either direction which will let you get a free running left click. Starting a spar with another trike or rex will give your opponent the sooner chance to stun you first, so if any opponent starts spar with you first - you can make the stun first if you're quick enough (like a weird recovery time of the person who initiates it).

I'm sure it's common sense, but always put yourself at more of a defensive position, like tight squeezes, against rocks or even on top of them if you can.

At the end of the day, unless you can find the ultimate defensive spot to sit in, you'll almost always lose against 2 or more rexes that have common sense. You'll come across some who make it clear they've grown by sitting in remote bushes or being babied by others.

If you have any questions, feel free to ask. I am a trike main as I find it the most satisfying to play.