Do I need to leave Thailand to apply for a DTV visa while on a tourist visa?

Yes, you need to apply from another country and stay there until approval.

Yes - you have to be in a diff country.

So, pick which country you are applying from, pre save all the details but leave out the location you’re at, then once you arrive at the country you’re applying from - update application and submit from there.

You’re meant to stay in the country you’re applying from, until the visa is approved, or denied.