Advice on camera choices

Hello r/canon. I’m looking into buying a canon full frame camera, my options are canon r5 or r6mk2, it’s probably worth mentioning I intend to buy used and there is a £500 difference in price. I have enough for a lens and the r6mk2 but i can just about stretch for the r5.

The dilemma is, is the r6 good enough for wildlife or is the extra mp on the r5 better value for money.

Another question is, how does the r5 perform in low light, as once a year I am asked to shoot a theatre show. I currently use my Sony A7R3 for this but it’s not really a good camera for wildlife as af is slow.

Thanks for your advice.