Nobody knows if you can renew a DTV after 5 years. They might make a decision in 2029. If Anutn gets his way about a new Immigration Department all bets are off for many visa types. Things change very rapidly in Thailand. Here is another Ed crackdown regarding course attendance
You can do it upon landing. They have terminals with helpers. I flew in Saturday but completed TDAC online. They did not even want to see it on my phone as it is all tied to your visa number they have on screen. If yiu cannot fix yourself go to one of the terminals upon landing.
Currently nobody is reporting have to show 500k upon entry or re-entry. Nobody knows if it will ever happen in the future. Deal with the situation as it arises. It may never.
You are the first one I have noticed from Laos. There have been postings from Japan and a couple of other places. If you went through agent it sounds like embassy sending message to agent
With 2 rejections, it sounds like you have a note on your immigration file in the system. You could use an agent with contacts to find out, and if so, what it says.