Hardware transcoding on SBCs is generally not fantastic, you’re gonna want to look for one that has VAAPI/VDPAU support or you’re gonna be looking at 100% CPU for half a day to transcode a film, which will make your other services effectively unavailable at the time.
I used to run my Plex server on a Pi4 with 4GB of ram and it basically crashed any time transcoding kicked in, I swapped to an intel NUC so I could get QuickSync for transcoding.
I’ll point out though, every SBC you’ve listed has usb, which is all you need for an external disk. If you’re worried about size, I’ve got a 5tb external drive that’s about 5cm², which is basically the footprint of any SBC you could use in this scenario