For photography(idk how it translate for video): It divide the MP by 2.56. Since it's a surface area, the crop is 1.6 per side. 1.6*1.6=2.56 So a 45MP R5 becomes roughly(because it's never exactly 45MP) 17.58MP My 24MP R8 becomes 9.4ish MP.
I've shot on the T6i for years, just got the R50 and I love it, definitely a step up and great way to get into the mirrorless space for a non-professional
Depends on the budget, but a Canon r10 or sony A6400 for 700-1000 USD will definitely get you going. But personally, I'd spend more on the lens rather than the body.