We wrote letters of intent for our children and it worked just fine. We actually had ChatGPT create a template for us and just filled in the required information like passport numbers DOBs and so on.
We just got ours approved through LA. Took about 3weeks then requested additional documents, then another 2 weeks. 3 weeks for dependent visas after that. It was stressful but only because of the waiting. $17k required funds, $400 per application, 3 months of bank statements, and just bio page of passport. We used the bank statements as proof of location as well as proof of funds.
My family is coming in on a DTV as well. For my husband they accepted 500,000 baht (soft power) and just the address of our air b&b in Bangkok as where we will be living. The air b&b is rented for just 30 days while we hunt for a permanent residence.
For me and our 2 boys we made sure we had at least 2x the money required to avoid any issues.
Here’s the thing though, we applied in the US and every embassy is different. Some are more relaxed, some more strict. Hopefully this helps a little.
We applied at the LA embassy in the US using a joint account in both mine and my husband’s name and got accepted with zero issue. He got the DTV and me and our kids are dependents