For the record I have not done this, I just want to see if it’s legal.
Would it break the rules for a player to modify the physical controller in a way were it can more easily do certain things (many holding 90)?
In my opinion it is pretty much impossible to verify the existence (or rather absence) of external modifications, hence it is an unenforceable rule, and therefore we have to rely on trust in this case. Sure, we could require some sort of face cam to show the controllers, but that a) might not be possible for everyone and b) still does not eliminate the possibility of a second unshown controller to be used.
So TLDR: I am personally not really worried about that and even if I were, it's nothing we can easily enforce.
Yes to what Pie said but also, if you stick with practicing holding 90 when you play it gets easier with time. It becomes automatic muscle memory eventually. If the controller modification made it so you couldnt hold full throttle anymore you’d actually lose time because holding 90 doesn’t start being advantageous until around 2700-2800 speed in most planes, before that you want full throttle.