Use Sneak attack + extra attack + Expertise in very specific skills (stealth, forgery kit, poisoner's kit, disguise, final). Some kind of half-fighter/half-rogue, like the paladin is a half-fighter/half-cleric.
Remove the Infiltration stuff from the archetype as it is something better left to roleplay. Or use them as ribbons.
I agree with the limited Expertise (stealth, sleight of hand, forgery kit, poisoner's kit, disguise kit,)
I was thinking less Extra Attack and freeing up bonus actions for dual wielding, steady aims, or cantrips.
I would think the Assassin could fit the Fighter/Rogue class some hoped for.I didn't even realize Assassin wasn't still its own class in 5e. It certainly should be, and is also the class that can fill the role of part-Fighter part-Thief/Rogue for those as want such.