I just got to level 8 and am currently Gloomstalker 5/Assassin 3. Surprise seems bugged though, it almost never triggers.
This. I went exactly that route. Many times you're infiltrating somewhere, you decide you want to kill them - then a cinematic triggers saying; "Oh, they'r eangry now' and you're not really catching them by suprise.
However. in instances where it *does* work, the thing you want killed, is dead. Still. I love my Gloomy.
Be sure to *start* as a rogue for the expertise in two skills [stealth, sleight of hand for me]. You'll miss out on medium armor and some weapon proficiemcies, but you'll have high dex anyway - And I'm an elf for the proficiencies.
However. Lae'zel as a pure Battlemaster has been the most consistently good character in my party. Bar none, not even my handmade pc. She's everywhere she needs to be with her psionics; has the most attacks out of everyone [six with action surge, currently at lvl 11] and hits like an absolute truck, too. I gave her the Sentinel feat + Riposte maximizing the odds of her killing stuff on her reaction. She is *such* a beast.
I made Gale a bard 1 / Diviner 10. Gale is..... Handy to have, and he would be missed if he weren't in the party - but he also doesn't shine. There's too many spells that rely on concentration and he has SO much trouble protecting himself. Misty step is mandatory. I gave him the Lucky feat, too - making his attackers reroll - but at the later stages that doesn't work. They only miss on a 1, anyway. Still, he's just a toolbox and the portent dice become important dice. There's only a handful of spells I find consistenly good - Magic missile, misty step and mirror Image. Everything else about Gale is situational. It's nice to have flihght and feather fall on you when you need it.