A dagger, any of the types, if properly geared can be a very viable option, and solo anywhere with an ISS as you described.
About spoil, it's nothing to write home about. The spoiler's other unique aspects like the coin buff, self pom, and mainly front facing play style make it unique. If you enjoy the class, I suggest staying with it.
All othels are viable as long as the are played correctly. The only class I dislike is advtenurer. The front facing aspect is done poorly imo. On spoiler it's done correctly. Wind rider is all about flanks, and the strongest pve dagger imo. Ghost hunter is all about rear damage, and shadow chase is very important.
For afk xp, get a mouse or keyboard from Logitech or razor and use a repeat macro to spam your dps skills. That is way faster xp than in game macro.
Also, don't use crit wound.
Edit:
End game fsrming toi solo, a spoiler would be very good there. The mobs are immune to status effects. Front facing setup of spolier theoretically should farm good, possibly best of all Daggers. I'd love a pts to test this sometime...