Regularly transfer much larger amounts with Wise, no problem at all. Usually in seconds, once in a while it takes longer but then they will state so in advance and you still have the option to cancel.

Got mine at local police station, not immigration.

This has been the rule already for a long time if you travel alone with one of your Thai children. Six years ago on KLM I was asked for the same letter when traveling with my daughter. They also took her alone without me present for an interview eventhough i had the letter and all papers. Fortunately all good and cleared with no problems but they seem to check more or less at random.

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.

You also need pictures of the two of you in front of your house and a few inside the house. No need to use agency, you can perfectly well do it yourself. I got the one year extension last month. Each immigration office is slightly different. In Udon they did not bother about aging of the money and all they needed was a letter from the bank that there was 400 000 bht on my account on the day I requested the extension.

They just visited our house today for the same. No charge. Just a friendly chat.

I aleays use Kasikorn, never any problems, online, and extremely fast.