It depends on where you are applying, as some Embassies require the balance for 3 months, and others only require the final balance to be 500k. I would supply statements from both accounts
You are correct, as the DTV is attached to your old passport. If your new passport has a different number, then carry both passports until you have time to attend immigration and have it transferred.
I wasn't saying keep the money in your home country, I was just advising not to move so much to Thailand. The banking system here is in upheaval, with many expats having their accounts frozen.
Thai banks have different limits on maximum transfers, so check the Wise website. Why are you transferring 1.5 million baht when you only need 400,000 for a marriage extension? My advice would be to not keep more funds in Thailand than is necessary.
Very good information. Just to add, the fine for not completing a 90-day report is 2000 baht. Currently, when you leave Thailand, the IO doesn't know if you did a 90-day report or not, which is why some DTV holders who leave every 6 months do not bother with 90-day or TM30, as they do not have to deal with immigration. I haven't tried this yet, but I will report if this is true.
For me, the biggest benefit of a DTV is that you do not have to have a large fund in Thailand, as the 500k can be in your home-country personal account. Once the visa is issued, you can transfer the money to a high-interest investment account. I cannot see an advantage to you in converting before your extension runs out.