Can I apply for a re-entry permit if I'm unsure when I will return to Thailand?

How about a complete sentence. You hadn't told us when you are leaving but you can return without a problem as long as it's before your visa expires

Immigration really should simplify this process. In Turkey, I received Citizenship by purchasing a villa for over $400,000 USD. I have never done anything else in 4 years. Granted Turkey is a screwed up piece of garbage of a country and I will be moving to Thailand regardless when I sell this villa. Still, Thailand Immigration is really ridiculous.

It's good until your visa expires

Actually not so long your EOS will not expire

Nope put in any dates that are inside the time of your extension, as long as your back before your extension expires it doesn't matter

You can go as long as you like but you have to return before your extension expires.