What do you have to report every 90 days to Santander. I have Santander current account plus savings account. About a year ago I had to confirm my address. That's all
They're not looking for income. You need 500,000 baht (or equivalent) in a bank account. If you're not working remotely, look at soft power options, eg. Medical, dental treatment, or perhaps cooking classes
As I understand there will be a couple of computer terminals available and assistance from an immigration department officer. I can only imagine how the queues will be for this, so I think anyone planning not to do it beforehand should allow an hour or two going through immigration, but at least your luggage will be waiting for you!