That's ok: with a dagger in one hand you use your Sneak Attack with one of those attacks.
It does occur to me that a straight up Assassin, at levels 17 and up, still has a (potentially*) vastly better alpha strike than any of these multiclasses. Not as much fun, maybe, but more damage. 11d6 doubled, then Dex added, then doubled again.
*It does require a failed saving throw. Almost worth dipping two levels in Wizard for Portent.
The saving throw requirement is the biggest problem. The value of a devastating alpha strike is to wreck the big boss monster at the start of combat... and big boss monsters are apt to be legendary, especially at 17th level, which means they can shut you down hard. Plus, it's a Con save, which high-level boss monsters are generally quite good at.
But even setting that aside, you actually don't gain much. Let's try a 17th-level gloom stalker build: Ranger 5, fighter 2, rogue 10. Using Action Surge:
First attack: 1d8 (base) + 1d8 (Dread Ambush) + 1d6 (hunter's mark) + 5 (Dex) = 2d8+1d6+5
Second attack: 1d8 (base) + 1d6 (hunter's mark) + 5 (Dex) = 1d8+1d6+5
Third attack: 1d8 (base) + 1d6 (hunter's mark) + 5 (Dex) = 1d8+1d6+5
Fourth attack: 1d8 (base) + 1d8 (Dread Ambush) + 1d6 (hunter's mark) + 5 (Dex) = 2d8+1d6+5
Fifth attack: 1d8 (base) + 1d6 (hunter's mark) + 5 (Dex) = 1d8+1d6+5
Sixth attack: 1d8 (base) + 1d6 (hunter's mark) + 5 (Dex) = 1d8+1d6+5
Sneak attack 1/turn: 5d6
Total: 8d8+11d6+30
Doubling all dice from automatic crits: 16d8+22d6+30 (average 179)
The assassin, meanwhile, gets 2d6 (dual shortswords) + 11d6 (Sneak Attack) + 5 (Dex). Doubling all dice from autocrits, that's 26d6+5, averaging 96. Doubling that is 192 average damage, a mere 7% gain. And it doesn't come online until 17th level, where the gloom stalker has been wrecking bosses since level 8.
Of course, this is assuming all attacks hit and the target fails its Con save; I haven't yet run the math on how those failure chances affect the total, but I think it favors the gloom stalker.