My kids school provided all the paperwork and the list of info the immigration would want I had to get separately like a tm 30 , a photo, and the correct form to transition from visa exempt to non ed. , photo copies of the passport including the info and photo page and any entries into Thailand. I made an appt online for immigration which was easy as there aren’t as many people trying to get the transition. We were in and out quite quickly and immigration gave us the extra forms we actually apparently needed. Unfortunatly the one year extension took an entire day of waiting around at immigration but the actual paperwork was super easy and provided by the school
Without a bank account you will have to leave the country and apply for 90 day non o there. Showing your home bank account amounts. Then re enter after approval… open a bank account here with the visa and season the money here, then apply for 1 year extension.
Also make sure you have an authenticated copy of your child’s birth certificate. In the United States, at least this is a multi step process that can take months and months 
If you truly have quite high passive income than the LTR retired visa would be better for you. You need 40,000 USD a year in passive income plus 250,000 USD investment in Thailand or 80,000 USD a year in passive income. Then you are good for five years, tax exempt in Thailand, check in once a year and no  reentry visas required.
using an agent can haunt you if you use them to fake money in your account but if you use them to make sure the paperwork is correct, and get you appointments, etc. then it’s likely worth it. We just spent an entire day at immigration in Bangkok despite arriving at 6:10 in the morning because the agents obviously had people who get in the front of the line and scoop up all the numbers. I was 191 in the pre-entry line but 215 once I got in I’ll never do that again. 100% worth whatever agent charges to not sit from 6 AM till nearly 5 PM waiting in line.
oh you can pick up and move, but might wanna hold off on selling the house or the car or whatever. I’m renting out my house, a friend of mine has my car, I resigned my job and took a new one, but I could get my old job back if I come back and I want it. My kids went to public school at home so if we come back, they’ll get right back in where they left off.
Most like Thailand. It’s an amazing place but… I can’t imagine putting my eggs in a basket I’ve never seen. That said, rent your place out… see if you can work remotely. Don’t burn bridges etc.