In my opinion, Solo's fate at the box office is mostly to blame on Disney themselves. Not because of any SJW bullshit or whatever, movie was perfectly fine and even good, but because they seemingly didn't really believe in it. The title was revealed very late, the marketing was nonexistent until recently, and most people I know don't even know it exists or that it is out.
The release window was also very badly timed since it came so soon after Infinity War (which is still in theaters) and Deadpool 2.
Add to that all the social media bullshit and the zillion articles and videos about every minute detail of every piece of drama surrounding the production and people had their mind made up for them before it even came out (for those few that even knew it was coming out since trailers were nonexistent until about a few weeks before it came out).
Hopefully, Disney can soak up the "relative flop" and move on to improve what didn't work without ruining the whole thing, but they've done fine by me so far so they still have my confidence. I sure hope they'll continue to release spinoffs in this universe because they really bring some new fresh blood to a saga that loves to run in circles (Rogue One and Solo are prequels so they kind of follow the rails of the story so far, but they at the same time use the smaller scope to show her things we never saw in a saga bloated with space wizards and family drama, thanks to them we saw the people in the galaxy a bit more, we saw how non-plot-armored rebels operate, we saw new places, we saw the criminal underworld, those things are interesting to me).
As for Finn in TFA and his janitor job, I always assumed he had regular trooper training but his reluctance to blindly follow orders got him in trouble with authority and landed him in sanitation. It would explain why he was so pissed at Phasma when he returns to Starkiller Base and is digging the whole humiliation part, since he'd consider it a fitting revenge for what she did. That's contradicted by Phasma telling Hux and Kylo that Finn never had any bad marks on his record prior to his hesitating on Jakku, but that wouldn't be the first time she's been shown to lie (including in the deleted scene of TLJ where her lies are revealed and she ends up shooting her own troopers to cover them up).