Should I get a tourist visa for Thailand or try to extend upon arrival while planning for a long-term marriage visa?

Have you considered getting a non-O based on being over 50 before coming to Thailand instead? It's probably the easiest visa to get and means you can open a bank account when you get here, so that you can extend your stay for a year. You can change it to a marriage extension the second year. Many people never bother changing to a marriage visa/extension (myself included) as it's a more complicated process.

If you come on a 60 days visa exemption or a 60 days tourist visa which both can be extended with additional 30 days is not important. You can apply for a Non O in country with both.

You’re problem is a bank account for banking the required 400K baht for Non O/extension.

As I see it you have two options;

-Just come and get married and contact an visa agent as

@THAI *****************************

who are able to assist you with opening a bank account included in assistance with the whole visa process.

- Come and get married and get all your papers, leave Thailand and apply for a Non O through an embassy in a nearby country with your bank at home to meet the financial requirements. Not a duckwalk but still possible to open a bank account in Thailand on a Non O visa and a residence certificate even though you might have to try in several branches first. Bank the 400K baht two months prior to your application for extension of stay.

Are you already married to a Thai? If so, you should get the non-O based on marriage before you go to Thailand.

If you already have a Thai bank account, you could do what you propose, but it's still not a good option because you'll have to go to immigration many times (4) and it'll take a long time.

If you're not already married and you don't have a Thai bank account, you're going to find things very difficult.

Go for a 90 days Non-O based on marriage applied for back home via eVisa.