I'm assuming you're referring to the yearly extension. 1) No 2) 60 days in a Thai bank account before your application date 3) 30 days before your visa or extension to date expires. Some immigrations offices allow to apply 45 days before last day. I'm not sure about Phuket's policy.
I've looked around for the same, but I feel there is not much that I would consider safe. The big banks offer very low interest rates despite high rates globally. You could open a brokerage account and put money into index funds, but better doing that with somewhere like IB or a brokerage in your home country. I do see some corporate bonds here that look promising, but I personally don't know enough about the Thai bond market. That's it from this armchair expert ๐
I kept my 400K in an e-savings account which doesn't have a passbook, so I submitted stamped bank statements instead, along with a deposit slip to show the updated balance for that day. It was accepted by Chonburi immigration, but the Bangkok office wanted it to show on the bank statement, so I had to do it over again.
I would keep 400K until you get your approval stamp. That means 3 months. This year my application required additional documents from the bank during the approval process, which required me to show (again) that I had 400K in the bank.