I won't comment much further on the Gloomstalker multi-class as it peripheral to thread, but I'll run that math again at level 8 too. We should assume no prebuffs and strict action economy, so if Hunter's mark is cast, poison is not applied. I can't remember the expect AC by level, but let's say it's something like AC13.
Round 1:
Glmstlk5/Assn3, Dex19, 3x rapier attack+7 @ d8+4 damage, +d8 ambush, +2d6 sneak attack, +d6 Hunter's mark. That's 75% accuracy to do 69 average damage (6d8+12+2d8+4d6+2d6), for an average DPR of 52 (rounded).
Assn8, Dex20, 1x rapier attack+8 @ d8+5 damage, +4d6 sneak attack, +d4 poison. That's 80% accuracy to do 47 (2d8+5+8d6+2d4), for an average DPR of 38(rounded).
That's a nice multi-class combo for DPR for sure. It's not that impressive overall though tbh. It looks like one of those power spikes.
Assassin dice are thereafter going to accrue faster for the single class, assassin 4 gives ASI whereas assassin 9 gives 5d6 sneak attack and Infiltration expertise. Not to mention already got Evasion, Uncanny dodge and 2nd Expertise.
But I tell you what. Both builds will perform nicely in BG3. And having a DPR killer-combo up your sleeve might come in handy for some unavoidable fight. I recall a video showing level 8 party fighting Kethric Thorm.
