Unless you have a long term visa when you arrive in Thailand, it is almost impossible to open a bank account here and a money in a Thai bank account is required to obtain a retirement visa herw. So best to arrive here with a long term visa. Then you can open a Thai bank account, season the required money and apply for an extension of stay based on being over 50.
It always depends on the Immigration Officer. There is no legal limit on the number of times that you can use the Visa Exempt Entry. However the Visa Exempt Entry is intended for tourists. Thai Immigration considers that 90 days is suitable for a vistor to stay in Thailand as a tourist. If you attempt to enter on a second Visa Exempt Entry, many Immigration officers consider that you are abusing the system and will take you aside for further questions. Depending on your answwers to these questions the officer will warn you that you will not be allowed to enter in future on the Visa Exempt or my deny you entry into Thailand. If you reseach these pages you will see that recently many have been denied entry for attempting to enter Thailand on their second Visa Exempt. If you want to stay in Thailand for more than 90 days, then to be safe, obtain a visa suited to the reason for your stay.
I do that. I keep my 800,000 + thb in one bank account and use a second bank account for my day to day use. Works well but Thai bank interest is crap low.
For Retirement visa, 800,000 thb in the bank and complete the copies and Immigration forms. You get the visa on same day as you apply as long as paperwork is correct. Different paperwork for each of the manyu Immigration Offices.