You could move your hub upstairs if the coax already exists, though would have to disconnect the tv box there... I guess if you have a splitter downstairs you could move that too.
The best answer is the one you have discounted already - running a cable - but that’s understandable, it can be a pain in the neck.
the next best answer is probably a wireless mesh system.. these come in various configurations and price brackets - if you’re looking for something that can provide reliable high speed and make the most of your 500mb then you’re probably looking at the higher end of the market which is £250+ check out the tp link M9 and the ASUS whole home mesh.
Powerline stuff is, imho, a last resort option..