hobbyist for my kids Mtn bike races - 70-200 RF vs EF, f/4 vs f/2.8
Over the last decade-ish I become somewhat of a hobbyist photographer but primarily for my own purposes of "journaling" my kids growing up. I don't have an unlimited budget by any means but I am able to buy nice-ish equipment that is not proportional to my level of skill or necessity (which is basically none.) Currently I have an EOS R that I've had since about the time it came out, and I have an RF 24-70 f/2.8, RF 35mm f/1.8, RF 50 f/1.8, RF 85mm f/2. My 24-70 is on my camera 99% of the time.
My older kids are on their high school mountain bike team and I am trying out different lenses because I'm not getting the background separation I want with the 24-70. I tried the 85mm last week and actually liked it quite a bit but it can be difficult with a prime and not always having a heads up on when they're going to pop out from the trees on the trail.
So - looking at a 70-200 f/2.8and unsure what to do. I was set on a cheap (er) EF (probably IS I, maybe II) with adapter, but even at $500 I don't want to be annoyed if it stops working and no one can service it. Then I started considering the f/4 because really if I'm going to be outside midday I probably don't need the f/2.8. And then at that point I thought "if I'm going to spend 700-800 on used and need to buy an adapter, maybe I should just look at the f/4 RF."
For my needs - daytime mountain bike races where autofocus and sharp at a higher shutter speed is everything - what is best and not overkill?
EF vs RF
f/2.8 vs f/4
Currently in my area prices are about $350 for EF f/4 IS, $650 EF f/2.8 IS, probably $800ish for f/2.8 IS II
$1200 RF f/4, and I think I'm unwilling to pay for RF f/2.8
thanks!!