This enclosure worked well for this, connected to a 1TB SSD Mac Mini 2018 (now 95% full) through USB-C:
Mediasonic ProRaid USB-C 2 Bay 2.5” SATA SSD/Hard Drive Enclosure – USB 3.1 Gen-II 10Gbps Speed/USB Type C (HUR6-SU31C Grey)
I put a 1TB Samsung SSD in bay 1 (daily_backup) and a 2TB Samsung HD in bay 2 (containing weekly_backup and monthly_backup). I formated both as APFS. I left the enclosure's default settings for 'single disks' as is (no raid).
The initial SD! clone to the SSD ran at up to 250 MB/s and took less than an hour.
The second Smart Backup to the SSD was mostly over 1GB/s and took less than 20 minutes.
I use TinkerTool System to set the three drives to not mount on startup, and SuperDuper! respects that by automatically mounting/unmounting the volumes as needed. Nicely, there is no light on the enclosure when the drives are unmounted (but plugged in), and I can tell by the feel of it that the spinning drive is not spinning then. [More on this: the drive instructions say the USB port of your PC must support power-off function so that the device would go to sleep mode, so this seems to be what is happening with this Mac Mini].