Should work okay, especially if you’re already happy with the existing external drive performance. In general they can be slower and more flaky though.
It sounds like you’re buying everything new anyway, and DAS enclosures can cost the same as a whole PC case. Consider a more standard PC/server build if you want the option of upgrading cheaply in the future. Search NAS motherboard on aliexpress, they have stuff that is basically low powered mini PC but with a load of SATA ports that can fit in a normal PC case.
Totally fair to use what you already have or can get cheaply and easily.
Yeah exactly. It would be a whole PC build, which I know you mentioned not wanting to do, it can be its own project selecting parts and putting it together. Can chuck it in a Node 304, or Jonsbo N1.
Wolfgang has a youtube video of putting a build together around it. https://www.youtube.com/watch?v=vjDoQA4C22c