In an ideal world, this scenario (I am in a similar income scenario) should be a prime case for the new LTR visa and it’s Work Remotely from Thailand category. Unfortunately, Thai authorities messed that visa up to a point that it is now only applicable to an almost non-existent group in that very category. If you have steady income from your UK limited, getting employed (with Thai work permit and long term visa) by an umbrella company through which you channel some of your invoices would be a way to go. Check out Shelter for example:
Unfortunately. Their minds are still quite far detached from modern times. Could be as easy as showing balance on a Thai banking app .... but it isn't.
Remember: Even having the 3 proofs, this does not guarantee entry into Thailand. They could still resort to other parts of the relevant legislation (mostly the Thai Immigration Act) to deny entry. Or they could just make requirements up that are not written anywhere. Sometimes, people got even straight to being shaken down without even being asked for any of those 3 proofs. The risk of denial increases with your travel history (and the length of stay of previous trips).
What the heck is going on today? OP talks about 90 day reporting and many people comment doomsday scenarios based on overstay. Read. Then comment. Cheers.
Do they really still do this locked phone thing in the US? That ended in Europe almost a decade ago, I think. Back then, you could jailbreak phones ... but that got harder to do, too.