Boost a thief?
Thief is a nice solo class, you have stealth that is really really nice at lots of places around the maps, its shortbow gives you lot of speed, you can output a lot of dmg and i just think that its fun, however he can sometimes feel like really squishy especially in the expansion maps etc. however a few traits and stats fix it ezil
Thief is great a killing things quickly, and has very nice damage. As far as comparison, the two classes are pretty much opposites.
Engi is a bit beefier, throws down turrets/explosives/shields and is very utility based, often using toolkit weapons like flamethrowers and elixir guns.
Thief is much more of an evasive playstyle, think hit and run. Level 80 content likes to throw big groups of enemies at you, so it can be easy to die if you try and sit in the fight too long. It may take time to adapt to that style but it is very fun and rewarding to play.
I highly recommend it!
Stay Low. Move Fast. Kill First. Die Last.
I think the biggest thing to learn about the thief is NOT to stay in a fight to long. You can sometimes feel you have someone on the ropes and press and this can lead to you dying quick. It not like Engineer in this regard. Keep mobile, keep aware and manage your INI.