Neat!

I'm curious about those hard drive caddies?

I found these a while back. They're brilliant -  and dirt cheap on eBay. I bought a load for 99p each. The drive just snaps into the plastic adaptor and then they're 3.5" bay sized. I used them here because they provide a bit of space for air flow and can be stuck together with sticky foam fixers. Here's a link on eBay, but I think you can get them on Amazon (for a bit more £) https://www.ebay.co.uk/itm/143478268428

Ahh, I thought they were 3.5" for some reason. Very useful though.

I think I 3D printed a 2 x 2.5" to 3.5" caddy a while ago when I couldn't be bothered waiting for shipping and wanted to fit some SSDs RIGHT NOW.

One thing on the Pi4 NAS setup: don't be a knob like me and format the NAS drives as NTFS. Raspbian has enough clevers to let it work just fine and is too polite to mention anything, but you will lose two thirds of your network speed and it will be generally much, much, slower, so that backups, big copy-jobs etc. take forever.

exFAT or ext4?

Well, right now I'm changing them to ext4.