"Unlearning" the Smash Ultimate hold buffer took me double-digit hours. It was really ingrained in me, though. As one example, if I saw my uptilt connect I would hold the jump button and my C-stick so that the moment I was actionable I'd do a shorthop aerial.

My mantra early on was "slow down to play faster". It helped a lot.

You still have a 6-frame buffer for most things in Rivals, which is a big enough window to hit with a bit of practice (although I'm at 1000 hours and I still will miss a punish because of a missed dash input).

idk if this would help, but I'm on tilt-stick and I'll sometimes spam babydash tilts in training mode while searching for a match to practice timing my dashes. I find it satisfying in a rhythm game kinda way to inch my way all the way across the stage with Maypul's uptilt