If you get a Yellow, your life will be much easier if you get get a CM4. The CM5 is still “early adopter” phase for that hardware, and it really isn’t going to give you anything over the CM4 for quite awhile. It’s easy enough to change out the CM board in a couple of years if you really want to.
If you do get a CM5 for the Yellow, absolutely do not, under any circumstance, use the screws to screw the CM5 to the Yellow. You will break the CM5. The screws aren’t needed, and they aren’t even mentioned in the instructions, but other CM5 users have used them anyway and reported that one of the screws cracks a resistor on the CM5 (the CM4 layout didn’t have this problem).