Except for the visa question, my advice would be to rethink to settle down in Bangkok. As exciting it might be at a first visit, it is not necessarily the greatest and most convenient place to retire, when there are so many other great places to start a new life with new friends.
I have the contacts of the sellers. According to the green books, the bikes are in my name. I understand from you, that all the bikes in my garage with my name in the green book are not owned by me? Who owns the bikes then? Please send us all a reference, to what you stated that I'm/we're not the owner without the 'contact'. BTW, we talk about Thai law in Thailand, not the law of the nation you are coming from. The same is true for cars. And yes, if you want to, ask the seller to sign a sell/buy contract, but it does not make a difference in future ownership.
I never got scammed when I bought 6 motorcycles over the years. Did you get scammed? I do not think it would make a difference to have a sales contract if you got really scammed.
I bought plenty of motorcycles with only the signed transfer document and signed copy of ID. Never got scammed, never had any problems. However, law/regulations might have changed. But let me know, where I can read 'the official' where it is stated I need a contract to change ownership of a . motorcycle in Thailand.
No sales contract needed. You buy what you see. Only green books get handed over and two signed transfer of ownership documents and copy of ID. That's how it is in Thailand.