It doesn't matter how much or how little your income is. That isnt a requirement. You just have to show a work portfolio or some sort or resume of your work
yes and you can open one if you have documented proof acceptable to the bank that you intend to live in Thailand. A real estate sales contract will 100% do this, as you need one to facilitate the title transfer and for paying land taxes
You don't need a bank account. There are many solutions available if you want to scan a QR code.
And using tap to pay using Apple Pay or Google Pay is getting more common now too.
For everything else, cash is king when it comes to local sellers. And its good to have some small change for tips or temple merit making donations. Keep tradition alive.