OP, you should pick whatever you want. It’s obvious from your cocky tone in the posts so far that you’ve got it all figured out.
You can’t articulate an actual use case. Just “I have some hardware, want to do things and stuff.” You don’t know how to install an OS without formatting over top of 700GB of existing data. Don’t know if you want headless or not.
How could anyone recommend an appropriate solution to a problem that isn’t even defined? Why was making a post here with no definition easier than watching one of thousands of tutorials on YouTube for installing a Debian based distro and then trying to figure out bash with to get by?
“I’m going to give Linux one final try…” Linux is giving you one final try. I’m sorry, you didn’t pass the test. Stick with windows homie. Just leave a windows desktop powered on for a long time. Congrats, you have a homelab now.
I’m not sure what you’re saying. You have a forced air system with a cold air return? What did you cut a hole into then?
Running the circulation fan constantly usually doesn’t help. If you have any ducting buried underground or in an attic (unconditioned), you’re just going to waste electricity using the fan, and dumping heat through the ducting into the attic or wherever.
Most systems have 3 fan settings 1) auto - run fan when heat or cooling are being called for 2) on - run constantly which is a waste 3) circulate - run fan for 2 minutes or so, every 10 minutes or so. Option 3 is what you want. But realistically “auto” is probably best unless you’re generating a few hundred watts or more in that one room.
A box fan in an open door will probably serve you better. Because it’s not pushing conditioned air through unconditioned space. It’s all staying inside the rooms and hallways.