Apply non o based on retirement before you travel that give 90 days on entry when you in the country open bank account and deposit 800k then take a trip to immigration ask for the list of requirements for one year extension and repeat to end of time. If you wish to leave the country buy a reentry permit to keep your visa alive
Apply non-o based on retirement before you travel give you 90 stay, then when you arrived start open bank and deposit the 800k then walk down to immigration ask for the list for 1 year extension, get that. Before you travel out get a reentry permit and when you return back to Thailand asks again immigration for the list and repeat
If you know your employer is going to give you the necessary paperwork for a work visa, it's best to go to Thailand, get everything sorted out there, and then head to immigration to get stamped in once you have your work permit with a work visa