Probably it takes months to solo lv 20 in eureka but at even in solo you can take part in NM and get some xp so in a way you are progressing,
That really isn't solo. The only reason those NMs are even showing up (and being beaten) is due to the efforts of other players, which you are then mooching off of. It's not like FATEs in the open world in that sense, which will just spawn on their own and can mostly be beaten on your own.