Okay, I did some digging.
I'd wait until lvl 3 for Mirror Image
Mirror image is invaluable for an assassin who got caught.
Gloom stalker should be good
Indeed. Improved initiative and extra attack/damage on first turn go incredibly well with an assassin type of character. Same for the increased movement speed. The damage bonus from Dueling and the extra attack at lvl 5 are also nice.
afais Gloom Stalker will have good synergy with Rouge/Assassin (sneak attack bonus, advantage on the first turn and critical hits on surprised enemies)
So the build plan would look like this:
Lvl 1-3 Cleric of Shar/Trickery Domain
Lvl 4-6 Rouge/Assassin
Lvl 7-12 Ranger/Gloom Stalker
not waiting until level 4 would delay her ASI...
Although as the party leader she needs to be good in charisma as well as in dex and wis, I try not to worry about ASI too much. Hopefully a headband of charisma or the like will show up soon enough.
So yeah, if this build is valid, there she is: a charming, deceiving, dark, holy assassin.
Edit: If the initial ability scores of the origin characters are unchangeable, then I'll probably respec once to dump int and str. Not exactly how I want to play, but at least the history of the class progression is unaltered.
Edit 2: Well, meanwhile I
do start to worry about ASI. In case it is crucial for the build, I'd put 4 level in each at the cost of the bonus action from Ranger.
Lvl 1-4 Cleric of Shar/Trickery Domain
Lvl 5-8 Rouge/Assassin
Lvl 9-12 Ranger/Gloom Stalker
The build seems more robust this way tbh. Thoughts?