I have been in Thailand for about 10 years with several types of visas, most were tourist visas. Not once have I been turned-away. I have been spoken to a couple of times but they let me in.
You never really know how its going to go. When Obama left the White House it was 36+, When Trump left it was 29.5. I figure 33 is the average, anything above is good, anything below - not good
Banks are kinda funny here, in my experience. Many times the bank customer must use the branch where they opened their account. Recently, I had to update my passport and I could only do that at the branch where I opened my account. I think this same is true for some/most/all immigration paperwork. Hence, your friend may want to open the bank account where they plan to settle.
I use agents frequently and recommend avoiding them if possible. They are great if you need them. Immigration can be a pain, it depends on where you are and the immigration office that serves the area. Agents make it easier but often times when you have a passport riddled with questionable visas, immigration will demand bribes (its an odd sort of demand).
However, in your situation, if you prefer to use the passive income method you will need to show 65K+ being deposited into a Thai bank account wholly controlled by you, over the course of a year. I suspect you do not have that requirement. Also, I believe the 800K method requires the money to be in the bank account for a couple of months before you apply for the visa.
You're best option may be to make a plan. Get a new tourist visa (or whatever), then execute your plan, and do whatever the next next time you need a visa