Take the ZFS plunge - My only real concern is the overhead

you shouldn’t worry about ZFS overhead if you are planning to use mergerfs.

you can tune the memory usage of ZFS significantly. by default it targets using half of your RAM, but on a home setup that’s wasting resources, you should be able to limit the arc cache to 1-2 GB, maybe somewhat more depending on how you want to use it. It’s done with sysctl or kernel parameters.