Looks very nice. I built a tiny PC to dump all my music CDs onto 42301.1 based around an ASRock STX motherboard. I imagine it's about the same size and, in the end, probably cost me almost as much as the much-more-powerful thing you've got. It was fun, and it did have space for the 2 SATA drives I wanted (along with an NVME drive), but I'd probably have been better going for something ready-made.

Looks a nice little system, hope it does the trick! Back when I bought my NUC, Ryzen systems were pretty much an impossibility to find unfortunately, still for a torrent machine, it does pretty well.

So far - so good (yes)

On the back of this, I'm upscaling my Alienware with a 1Tb SSD in place of it's current 1Tb HDD.

Presumably I can just take out the HDD, insert SSD and then install Windows using the USB thingy that I got from Dell for clean/re-installing/repairing Windows 10?

Yep, should work fine. And of course if anything goes wrong, you can always pop the HDD back in temporarily to resurrect the system.