Being uncertain about your departure plans, and just giving the maximum days, whilst unsure about this number, certainly gave the impression that you are just trying to stay as long as possible. If you had shown a return ticket with a booking within 30 days I think you would not have had any problems. Impressions count.
Retirement visa is the easiest, if you are over 50. Marriage visa is a bit more cumbersome but has additional benefits e.g. you are not prevented from working.
I opened a bank account when I was working in the UK, 20 years ago, moved to Asia 2 years later and still have my bank account in the UK, transferring money to and from Asia.
If you have an account in the UK and move to Thailand, there is no issue keeping your account. Give your Thai address to your bank and they will send all correspondence there. Once you have an account in Thailand you can use Wise to transfer money from the UK.
you have to use the main immigration office near the airport. I suggest you go to the immigration office before preparing the paperwork. Requirements vary from one office to another and from year to year.