I had found that straight thief to be fine with the fast hands ability to disarm a trap or open a lock to be useful in combat sometimes. Trickster was ok, mostly at higher levels when you can use mage hand to gain advantage on attacking. The spell selection was mostly used for spells like shield and invisibility.
The feat mobile is good to gain the 10ft extra movement and the free disengage after you attack. Better for a halfling as well to gain the speed, but also to save your cunning action to hide. Alertness is taken a bit by one of my players for the increase to initiative. I think some of the reason is to go first in combat and assassinate.
Don't forget the aim ability that came out to gain advantage if you do not use flanking rules. 2-weapon fighting is also good to make sure you can get backstab to deal damage.