This has been on my list of things to try for a while, but I’m currently in “if-it-aint-broke-dont-fix-it” mode.

I have one of these. It’s usb 3.0 only which it sounds like is what you have anyway. It’s not part of my NAS, I just use it when I need to quickly look for something on an old drive, but it’s been pretty reliable for me for the last few years and was cheap, so I have no complaints.

Ah my mistake. As I’m sure you’ve found you can certainly get USB-SATA adapters. They’ll be $15-20 each, so you’d realistically be better off getting 2 2-drive enclosures since that would be about the same price but much cleaner. I’ve used Sabrent for this for a while and they’re fine. There are occasional usb disconnects so it’s not good for anything mission critical. And never do anything port-powered when connecting drives over usb, always use parts that get their own wall power.

Buy a cheap LSI card on ebay, they can usually be had for around $15-20. Make sure it’s either in HBA or IT mode, or that there’s a way to put it into that mode. If it’s in HBA/IT mode, you can then just use it like more SATA ports. Buy a pack of LSI-SATA cables (there are two kinds, get the kind that includes the SATA power connector). Then you can put the card inside your computer and the drives anywhere that the cables will reach.

There’re a lot of keys on that keyboard…

I’ll second the kobo ecosystem but add that if you install the 3rd party koreader software on it it will make syncing to a local calibre-web seamless. And it leaves the stock reader software in place so you can read drm encrypted files if needed. I have a similar use case to yours where I use the device to read books but also papers, which I usually pdf and put in a specific shelf in calibre