You can use monthly income under a marriage extension, but it might be difficult for your initial visa and first extension. But from the 2nd extension it should be pretty smooth sailing if you've got a monthly deposit of at least 40k.
Monthly income for farang men need to be 40k or for marriage visa.
If the reason is marriage, AND you work with a work permit and make the minimum required income. Then you have a path towards citizenship. You can skip Permanent Residency too.
IF you are in the position of being able to and wanting a job in Thailand, a marriage visa will allow you to get a work permit.
This opens up a path towards citizenship for you.
The retirement visa does not do this.
You need more money in the bank for Retirement.
for Marriage visa you just need to show proof of marriage, proof of finances, some weird and somewhat intrusive photos of you as a couple( or family ) in various places in your home and whatever else your specific immigration office requires you to deliver as documents.
Shouldn't really be much harder than a retirement.