You could come over on tourist visa and open a bank account, but keep in mind you will not be able to get the visa or extension until you have the 12 monthly transfers
The best approach is to apply for OA visa from your own country. You use home country funds for this. When you get to Thailand start the monthly transfers. You cannot get a visa unless you already have 12 x monthly transfers
No. You still have to satisfy the full conditions for each separate extension. I did it differently by initially getting an OA visa, which effectively gives you two years and requires zero money in Thai bank account. I started the 65k transfers midway through the first year, so by the time I applied for my first in-country extension I already had 18 months of transfers
baht from overseas into your Thai bank account at monthly intervals. When you apply for extension you just have to show these foreign transfers in your bank book or on bank statements to immigration and this is accepted in lieu of the 800k lump sum