You can not have the money required at immigration split up between 2 banks. At least, I have never heard of that before. What did your parents know about Thai banks when you were in your teens?
I think you have misunderstood the whole thing. Bangkok Bank freezes the money 4 months prior to the day you apply for an extension at immigration to get the bank letter. The difference is just 2 more months compared to the requirement from immigration.You never switch from Bangkok Bank to Krungthai. That's not very smart. If you move money from one bank to another,it must happen the same day. Otherwise the time the money have been in the new account starts from that day. I hope you understand what I mean.
It does not work everywhere in Thailand. Especially changes between regions. When it comes to banks, requirements and regulations,Thailand is not a "modern" country. You are a bit naive or you haven't been around.
No,not plenty. Australia stopped issue income letters 2019 January 1. But my comment was about Europe which I guess you don't know much about when it comes to embassies and consulates.