@Bonnie ***********
You should enter Thailand on a 90-days Non-Imm-O Retirement Visa which you have applied for and got issued outside of Thailand.
*** On this visa-type, you definitely can get a bank account opened on your own, and transfer the required minimum deposit of 800,000 THB into it.
The process (at least on most banks) includes you must get a “certificate of residency” from Immigration, asking the bank to open an account for you. You also need a Thai mobile phone number and the SIM-card registered to your passport number and your name.
*** For the application to the 1-year Extension of the Stay Permit based on Retirement, you have to be registered in your accommodation by the TM30 online system of Immigration.
No rental contract is needed (at least on most Immigrations).
*** You will have to fulfill the financial requirement, which, at least for the first year, is a minimum of 800,000 THB in your Thai bank account. On the day you apply for the 1-year Extension of the Stay Permit, it needs to have seasoned for a minimum of 2 months, but you do not have to prove that the deposit came from abroad.
*** On The day you apply, you visit the bank, withdraw a small amount at the ATM outside of the bank, keep the paper receipt, then get the passbook updated and ask for the “bank letter of guarantee” (in Thai: rab roong thanakan)
You will need the following, and each copy signed in blue ink:
*** copy of your passport detail page
*** copy of the Non-O visa
*** copy of the entry stamp page
*** TM47 and TM7(you will get the form on Immigration)
*** a signed STM2 form (on Immigration)
*** 1900 Baht in cash
*** bring a few passport size pictures, you might need two pics when you buy a re-entry permit for the 1-year extension on the same day