Do as told, apply for a 90 days Non O visa at the embassy at home or buy a cheap onward ticket within 60 days. Cheap insurance anyway when you risk to lose your whole trip.
If you fulfil the requirements for and apply for a 90 days Non O visa before you leave, outbound tickets it’s not an issue.
For both tourist visa and visa exemptions an onward ticket is formally required within the initial 60 days even though you can apply for and almost certainly will get a 30 days extension.
I don’t advise anyone, just stating the fact that cash still is the formal requirement, BUT some also are allowed to prove funds in other ways. I guess this as some point will become more formalised as cash is no longer king in Thailand, but it’s still the formal requirement.
Cash is still the formal requirement, but based on feedbacks some are also are allowed to show print of bank statements or bank app to prove sufficient funds.
There’s no official restriction and totally in the discretion of the IO you’re standing in front of. If they at one point means you’re misusing tourist solution for long terms stays in Thailand you’ll be told to get a proper visa.