I was there last year never got asked for proof of anything and my return was 3 months after arriving,my airline only asked if I was leaving part way through my stay but didn’t ask for proof I wouldn’t overly worry about it tbh
I went for 90 days (during 45 day free stamp on entry) no flight booked of thailand,i stayed 44 days then flew to bali for 10 days then back to thailand for the rest of my 90 days visit....no questions asked at all no problems
Just carry 500 quid in gbp that will cover the 20,000 ive never been asked in all the years ive been there plus i always take cadh never used a card cos they shaft you then ya own bank shafts you as well for the pleasure!!