I have one of these mini PCs arriving soon. A little more money than I wanted to spend and no idea if this brand is any good. But it checks a lot of boxes for a backup server project I’m working on - eMMC for boot, microsd slot, M.2 2280 NVME socket, and a 2.5" SATA connection. They have some other variants as well that don’t cost as much.
https://www.amazon.com/dp/B0B6P4MDWN
I originally got a used Dell Optiplex Micro from eBay for this. They come with a 2.5" SATA connection. But I learned the hard way that not all models come with M.2.
Simplest solution is often the best. To add to this, Thunderbird has what are called Local folders. If you have your other email accounts added, you can simply drag and drop everything into the Local folder to move them off the cloud and into local storage. No email server required. Definitely a good idea to take backups if storing important stuff locally though.
Somewhat related, I’m so annoyed that my USB 3.0 SATA adapter doesn’t pass S.M.A.R.T. data. Didn’t even think to check for such a deficiency. Just another thing to watch out for when buying these adapters.