if that was your 2nd land border crossing for a 30 day visa exempt stamp that’s all you can do in a calendar year. (That’s why they said you must fly in if you want another 30 day visa exempt stamp)
And yes each 30 day visa exempt stamp can be extended by 30 days for 1900 baht at a immigration office in country.
Seeing as the group is “Thai visa advice” I’ll hold back on the relationship advice. That’s your personal choice.
Talk to a lawyer specializing in company setups. With the amount of money you would be investing to buy into a restaurant in that area and the number of staff you’d be hiring you would qualify for a non immigrant B visa.
I think the letter from the bank just says the the account is in order and the money has been in the account for the required time. Nothing to do with coming from overseas.