So after struggling for months to try to shoehorn a reduced version of my NAS data to a USB HDD, I gave up and decided that the only futureproof, comfortable solution to backup the NAS is to another NAS.
@timaras Why not use this drive for eg?
https://www.amazon.com/gp/product/B08XKL3GF9/
(btw I got it on Black Friday, last November, and now it's cheaper!)