Do I need to maintain 800,000 THB in my account, and how can my daughter inherit it after I pass?

Get a Will

You need yo talk to your local immigration to clarify the rules and a solicitor in thailand for your thai assetts....

If you start transferring 65k baht a month you will be able to use that for your 2nd extension but the money in the bank has to be there for your first year

Make a will locally, there are rules on how long the actual 800k needs to be on the bank think it’s something like 3 months before 2 months after, if you can just put it in a term deposit and forget about it, interest at bkk bank is 1.6 % so you get a bit more than 10,000 after they deduct the tax, rubbish of course and if it’s locked up you can never fuck up.

If you get the non-O from outside of Thailand then travel to Thailand, in order to apply for the 1-year extension the 800,000 must be in your account for 2 full months already on the day you apply for the extension.

Once you receive the extension, your bank account must remain at 800,000 for 3 full months, and then it can never drop below 400,000 the rest of the year.

Next year, you must bring it back up to 800,000 for a full 2 months before you will apply for the extension.

Repeat every year. It's much easier if you just leave it at 800,000 all the time so you don't have to worry about when to deposit the money to bring it back up to the required amount.

The money is not for your living expenses, it is for emergencies to show the government that you will not become their problem if something bad happens.