Just use Skyscanner or Kayak or any search engine to book a cheap one way outbound flight. I just booked for myself from Bangkok to Phnon phen cost me £30
If you try to enter Thailand on a 30 day visa on arrival and your return flight is after the 30 day period, you may be denied boarding by your airline. I run a golf group in Pattaya and since covid airlines have been checking visas more thoroughly. Either get a TR ( 60 day ) visa before leaving or buy a cheap flight out of Thailand to somewhere like cambodia or similar.
Ihave been back on the Thai Embassy website and it was the Long Stay visa ( O-A Non Imm ) that last year asked for a medical policy ( for the first time ever ) whilst in Thailand. However, it is now removed for the one year visa but still therefor the 5 year long stay visa. So I will now be applying for the 1 year long stay for my return in October. ( Glad i checked it ).