Thank you both, I feel knackered a bit better after some sleep.
@ruralwanderer the devices you have directly affect the cycles you're invited to. Generally, cycles will either be android 4+ or android 6+, I find, but then sometimes slots are given for specific devices. In terms of iPhones, iOS 9+ is generally required.
When I started, I just had a Samsung S6 Edge + (android 7.0) but it was really getting slow, so I upgraded to a S10. That's been totally worth it. Then I got hold of a couple of old phones my parents had in a drawer (A3 and J3), they're android 5.1.1 and android 8.0. I also took my daughter's knackered old tablet which is android 4.1. It's really useful because sometimes the old devices throw up bugs that the new devices won't.
In terms of iPhones, I got a second hand iPhone 7 plus. It's doubled the cycles I'm invited to - sometimes I get invited to 2 cycles that are exactly the same but one android and one iOS. Having said that, there's a quite lucrative project right now that I was invited to but can't do because you need iPhone 8 onwards, but that's rare. The iPhone 7 plus cost me £150 and it paid me back within a week or two.
Having both iOS and android is very useful. For example, I'm on a cycle right now that wants to knows if a bug can be reproduced on android or iOS and vice versa. If I didn't have both, I'd have to rely on someone else to perhaps check. As it is, I can say 'this reproduces on iOS' or 'iOS is fine', which increases the value of my report.