I have access to my account at all times. And you are correct I don't get any interest on that amount , however it's there incase of emergency and the rest is held in another country.
When you apply for the none "O" visa at your Thai embassy in your home country you need to show thar you have the required amount in your own currency and has been there for a minimum of 3 months.
Once you arrive in Thailand with your "O" visa then a Thai bank will open you an account.
With your "O" visa you'll get 90 day's which you must open a bank account then you extend those 90 day's to another 12 months.
So deposit your money ASAP and you'll get 15 months on tour first visa.
90 day stamp is on paper that's stapled into your passport so why are you running out of room ???, or if you don't want a piece of paper in your passport then do your 90 day reporting online.
To get this my English wife holds a none O visa based on retirement.
We then had our marriage certificate verified by the British embassy and then translated into Thai, which was then verified by the ministry of foreign affairs which was all done whilst being in Thailand.