There is NO OFFICE anymore that has the 'must file a 90 day report' before you get a certificate of residence
Bangkok USED to require it, they do not any more. All you need is a valid TM30 showing you're registerered as staying in Bangkok and you can apply for a certificate of residence at the main immigration office at Chaengwattana Government Complex
you can enter/exit thailand as much as you want during the validity of the visa itself (you said yours is Sept 2025 to Mar 2026) Each time you enter you get a 60 day entry stamp. You can extend each entry one time for 30 days for 1900baht at the immigration office if you want to
When your stamp runs out you need to exit/re-enter the country and you will get a new 60 day entry stamp <- you do that as long as the visa is valid for.
you should have no problem changing from marriage extension to retirement
You're right it's a LOT less paperwork and you get the retirement extension issued the same day you apply rather than have to go back in a month to get it inked in
Between now and when your extension comes due just ask your office for the hand out for retirement extension.
Most offices are only too happy to have to switch because it's less paperwork for them too 😛