Can I apply for a marriage visa while in Thailand after initially entering on a Multiple Entry visa?

The TM-60 multiple entry visa can be obtained in your home country. It's not an option within Thailand. You can change to a Type O or OA visa after arrival.

Your plan is sound. The initial Non-O marriage Visa is a 90 day stamp and the 12 month extension is repeated every year (you purchase a re-entry permit separately if required). Both require a Thai bank account in your name with B400k at the time of application for the first year. Subsequent years can use the B40k "income" method.

Getting married in Thailand is a fairly easy process as long as you are near to or stay in Bangkok for the Embassy visit and for the Ministry of Foreign Affairs visit. Depending on the Amphur where you plan to get married at it can be achieved in about a week without too much stress. All of the steps are listed here -

Get the Non Imm O, either based on marriage if you're legally married or on being over 50 if you want to go the 'retirement' route.

@Stuart **********

agree, that gives you a period to determine as you say " if current relationship remains as is ".

My experience only, so your mileage may vary: I arrived on a 60-day visa (tourist), submitted an affidavit certified by the embassy declaring my eligibility to marry, certified my passport at the embassy, had an agent translate documents and obtain a document from the Bureau of Foreign Affairs, went to the local district office (amphur), and registered the marriage. I then applied for non-immigrant "O" visa with all of the required documentation. Theoretically, the application is supposed to come from the Thai Consulate in your home country or one of the countries adjacent to Thailand but we managed to get around that. I wouldn't change a thing on the way I did mine because it best suits my goals. The non-immigrant "O" based on marriage allows for work permits and work where it is forbidden under the retirement visa. The bank requirements are also less. My balance before renewal has to be 400,000 THB vs retirement 800,000 THB OR if I opt for the monthly recurring deposit it is 45,000 THB vs retirement 65,000 THB.

If you are over 50 it might be easier to get the non o based on retirement. You can change it to marriage extension the year after if you want to.

As long as you get married in Thailand before your last entry stamp expires on your visa then you can absolutely apply for the 3 month non-o visa based on marriage while in Thailand. You'll likely want to give yourself a couple of months after getting married to get all the paperwork and requirements gathered. Prior to the initial 3 months being up you then apply for the 1 year extension. Then repeat every year for as long as you want. It's exactly what I did a few years ago.

for the application to a Non-Imm-O marriage visa in your home country, your original marriage documents would be accepted. Only for the application to a "change of visa type" (from a tourist visa to a Non-Imm-O visa) on Immigration to the Non-Imm-O family visa and the subsequent one-year extension, you will need the marriage to be legalized and registered in Thailand, because Immigration will want to see a freshly printed Kor Ror 22 - the "Thai rose bordered marriage certificate". You also would need a minimum of 400,000.- THB on a Thai bank account in your sole name. For the "change" to the 90-days Non-O visa, no seasoning is required, the money just must sit in the account and you need to prove it came from abroad. Only when you apply for the 1-year Extension of Stay as soon as 30 days are left of the 90-days stay permit, the money must have seasoned for two months and your bank must issue a statement.

Your plan is fine.

You must be legally married before applying for a marriage visa from outside of Thailand or for an extension of stay based on marriage from inside Thailand.