I think you have done the easy stuff by having a decent modem, looking at your wiring etc
I had the same issue, living in the countryside but wanting to access my network whilst away from home makes low upstream bandwidth super annoying
I ended up getting a second line and using dual-WAN router (without bonding) which is a sort-of halfway house, albeit pretty expensive.
I also found that my upstream bandwidth was being capped by my ISP because when I upgraded to "unlimited" my upstream bandwidth doubled.
Recently BT kindly installed a new cabinet close to my home and suddenly both my connections instantly went to 80 /20 which is nice, so I can now get 40 Mb upstream which helps a lot when I am uploading files, hurrrah!
C