how much of the internet is unusable with js disabled
Quite a lot actually. A lot of articles / blogs / news sites are actually more usable without javascript than with, because none of the annoying popups and shit can load. I suggest having two browser profiles: one with javascript enabled by default, and one with javascript disabled. So for things like online shopping, you'd open the js profile. And for things where you expect to do a lot of reading, use the nojs profile. Ublock origin also lets you temporarily enable/disable js for a particular website pretty easily.