TrueNAS might be a good middle ground then. Haven’t used it myself but I’ve heard a lot of good stuff about it (and it’s predecessor).

It really comes down to personal preference. A pure Linux distribution for example is certainly more flexible and will absolutely do everything and more that TrueNAS does. But a nice gui where everything just works and does exactly what you need sounds x100 times better than spending a whole weekend trying to fix something that you borked by accident. And if you want to learn how things work under the hood later on, you can do it at your own pace. At a certain point those terminal commands will stop being esoteric and start making sense.

If you have some old hardware just lying around, then install it and try it out. Just dip your toes and evaluate for a few days/weeks.

Also - after some quick googling. Mounting your windows disks shouldn’t be a problem. So no need for some temporary/intermediate storage unless you want another backup.