What are the best options for providing proof of onward travel to Thailand without issues?

flown into Thailand many times on a 1 way ticket, if you’re re genuine, you should have an issue

Use the words that thai immigration uses. Im sure its not "exit flight required".

If airline check-in wants proof of a flight out, show them the Thai immigration law requirements for exiting their country.

Few years ago while doing a check in (longhaul heathrow) i had a one way ticket with no proof of exit in any form of transport.

I already knew I could request a disclaimer, stating I would be financially responsible for return journey if immigration refused me entry to their country. Within 5 minutes check in desk had printed off the form with all my personal and journey details and had me sign it.

Of course immigration didnt even ask for an onward ticket but it kept the airline happy!!

Malaysia air do fully refundable tickets and the refund comes through in about 10 days, but it's a full price ticket

You have to make sure you buy a fully refundable ticket.

Only your airline can tell you what onward travel ticket they accept because it is they who are likely to ask for this proof