I had 800k equivalent in my home bank account. There was nothing about the sponsor letter. Use the financial requirements, quick and no mucking around.
mine the same. Opened it 20 + years ago. I had never been asked to update anything either. However on a recent branch interaction the lady did mention to have my phone verified just as a precaution. No drama, went to dtac and got a print out with my name and passport number on it.
It avoids you having to pay a 250 baht atm fee + any extras your home bank charges you. I do cardless withdrawals and transfers within Thailand and there are zero fees.
They are not going to close you down for buying a new phone. I just walked into the bank and said, this is my new phone, can you help me set it up? And they did, didn't asked me anything else.