@tuga - for a couple of days (whilst waiting for the M1 to arrive) I tried running a Samsung Evo SSD in a dock, using it as the boot drive and host for the Mojave OS.
Once I got my head round the recovery disk/OS install thing, it worked surprisingly well. Of course it won't be as quick as an internally-mounted SSD but the difference compared to the tired old 1TB HDD inside the Mac, it was like night & day. This was just using a USB connection, but as a stopgap I'd have been more than happy.
