A Win 8.1 Pro key will not work. Unfortunately, if you don't have a license key for Windows Embedded 8.1 Industry Pro specifically then you won't be able to use it beyond the evaluation period.
I installed it in a vbox vm, running on my w530, using an iso downloaded from
here (after verifying the sha1, of course). It's already showing the "activate now" watermark, so the evaluation period may be only 3 days. It definitely has a smaller disk footprint than win 8.1 pro, 16 GB vs (from memory) 24 GB, possibly due to there being hardly any apps installed. That's after reducing the paging file to 1 GB and turning off hibernation. You mentioned the lack of telemetry, but
this works fine to remove that from win 8.1 and Win 7. It's a shame there's nothing similar for win10.
Anyway, thanks, it was interesting. One thing I had forgotten is just how absolutely terrible the pre-win10 windows update had gotten. It took multiple windows update sessions to get it fully updated, and each took exponentially longer that the last to complete - several hours at the end, and running disk cleanup to get rid of the old windows update files took nearly 5 hours. The vm has 4 cores and 8 GB of ram, so resources aren't the problem. The w530 runs Fedora, has 32 gigs of ram, and is wonderful for running vms.