This is interesting, because i’ve done very similar with my old Pi2b, though i did the Windows side of things in the GUI. Backups are painfully slow, because the USB bandwidth is shared with Ethernet on all Pi pre the new 4 series.
I contemplated documenting, or even making a video of the process… but you know, stuff gets in the way. I do have some doco, but it’s a mess. I’ll clean it up just in time for it to be useless