Enclosures made for SSDs are usually fine. I've been using one from
OWC that holds both a m.2 stick AND a traditional HDD inside ONE case with a few extra thunderbolt ports for more than a year now. It gets a LOT of fairly intense use as a video editing scratch disc(s). No problems. OP, if you like the one you identify in post #1, be sure to look up reviews to confirm it is a reasonably good one. If not, shop around. There's plenty of enclosure+hub combos available.
However, as others offered, I wouldn't be trying to double up daily use storage and TM storage. Keep those
separate.
Since you seem to have an aesthetics (hide the drives) goal, look into putting your TM drive elsewhere. Does your router have a USB jack? If so, perhaps you can hook the TM drive to your router and turn it into a network TM drive? That generally works very well and can hide the hard drive (visually and audibly) elsewhere (where you keep your router). Bonus: if you are a multiple Mac household, others can backup to it too (a shared TM backup drive if you will).
OR perhaps look into adding a dedicated NAS drive for TM to do the same? It will have an ethernet port and you can connect it to the router by ethernet. This can work well when the router doesn't have a USB jack. Just use one of the ethernet ports. For example, here's a
4-star-rated 6TB NAS drive for about $200 and there are plenty of fish in that sea with all levels of backup storage available... including multi-drive units for gigantic storage if desired. I chose the latter using Synology enclosures myself. Same bonus (of being able to backup multiple Macs if desired here too).
BOTH, meaning use both types of drive for TM:
- the always-attached NAS drive for regular, ongoing backups AND
- the direct attached drive you already have for occasional TM backups.
Then, store the latter OFFSITE so that if your Mac and home TM drive are both lost (fire, theft, flood, etc), you still have a recently updated TM drive to recover everything up to the last day you updated it. I use a 3-drive approach myself:
- always-attached NAS for daily TM backups
- one DAS drive at home for regular backups
- one DAS drive stored offsite in a safe deposit box to swap with #2.
When I'm ready for an approx monthly swap, I TM backup up to date to #2, then take it to the bank to swap with #3. A few weeks later, I'm ready to swap again and repeat the process. At a monthly swap pace, worst case scenario is losing the files not backed up to #3 since the last swap. If there is anything particularly important to not risk that scenario, I can simply do a swap earlier than monthly.