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 How much is your data?
@sosonis about 20TB, from which about 15TB needs to be backed up.

@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!)

@sosonis I may end up in the same situation that forced me to get a Nas in thr first place - it doesn't scale easily or offer redundancy