Apols for the wall of text.
I'd hazard the most direct equivalent of the 750D would be the R10. R7 would be the upgrade (crop), R8 the equivalent of the (full frame) 6D/6DII.l, R6/R6II something akin to the 5DIII/IV - all ish obviously, not exact 1:1 swaps.
All would need a mount adaptor to use your EF lenses - but simple enough to leave that attached to the body full time as I do with my R6II. My pal tried a non-Canon one, he now has a Canon one.
Obviously the crop lenses won't perform on a the full frame body... On an EF FF body it wouldn't work at all because it would foul the mirror. On an RF FF I'm not sure what it does but the EF-S lens image circle will definitely not cover the FF sensor.
I've only got hands on experience with the R6II & R7 and they're both massive advances from the 90D & 6DII - not least the massive number of frames you can squirt off in a second & before the buffer fills (I've yet to reach the buffer limit on the R6II, R7 belongs to my pal - so I've not heard him swear at it for that). The tracking focus can sometimes struggle in busy backgrounds on both, but manually (on a 90D/6DII) you be even more challenged in most cases I reckon.
The only thing he thinks is less than great is his Sigma 150-600 - seems to hunt focus a lot. My only gripe is the Canon 100mm f/2.8 non-L which seems to make a lot of (apparently) IS type noise... I suspect that is true of all of them, and that one simply sticks out particularly.
Oh and if you're (she's) a considered shooter (look, frame, look, look more, shoot) instead of a press & squirt shooter, because the viewfinder is essentially live view full time, it canes the battery. Extra batteries a must in my opinion (I'm trying a Neewer 3-pack of LP-E6NH on my R6II, so far with positive feelings - 3 for the ¾ the cost of one Canon)
Oh & fastest card that the camera will take (and you're prepared to shell out for obvs) makes a significant difference to the buffer performance. I ended up with a Kingston Canvas React Plus based on performance & price. YMMV obvs.