How do I apply for a marriage visa at a Thai embassy in my home country and fulfill the THB 400,000 security deposit requirement?

The 400k in a Thai bank is required to extend an existing Visa while in Thailand. Last time I got a Visa in my country they required me to produce a bank statement with a minimum balance of 6000 Euro (single entry non O). The last time I applied for a Visa it was in Ho Chi Minh. I payed 200 USD and no fund proof required (multiple entry non O). All pre covid, things might have been changed meantime

Yes Singapore i had to show 0 baht or equivilant in Bank account, account could be from any country, there were three of us last July first to apply after re opening and the three of us all showed accounts from different countries and were accepted.

Pretty much as Geert says above and also things can differ from each immigration office. Just to add, once entered on your Non Imm O, that lasts for 3 months but you can apply for your extention based on marriage (form TM7) early. I think upto 30 days before it runs out, so you get plenty of time. Open your bank account straight away.

That is not how it worked for me. In your home country (Holland in my case) you apply for a standard Non-O visum (valid 1 year and allows 3 months in TH). For this, you only need to prove that you have 'sufficient' income, for which 3 payslips or 3 months worth of (Dutch) bank account data is sufficient. You do not need to have a Thai bank and no need for 2 month seasoning. Next you enter Thailand on your Non-O and once in TH you get yourself a Thai bank account and deposit 400 000. Then next you go to immigration and request a one year extension, that's quite some paperwork but it's doable, no big deal. Once your application is received you get an 'under consideration' stamp which once again allows you to stay for 3 months. Basically, that's seen as the 'seasoning' period as after this period, when you have to return to immigration once more to get your actual one year extension, they might ask you for your Thai bankbook once more to see if the 400 000 is still there (they did not even bother with me, but I was warned they might ask). So first get your normal 3 month Non-O in your home country, and then get all the other stuff done in TH. But once more, sadly it differs per country and per immigration post.

Never marry, only book, the Cheapest way and can Change fast.