Survivability and the ability to engage effectively matters more than DPR. That's the whole premise of the ranged superiority argument, nicht so? The game devs kept a tight rein on offensive abilities, and crossbow expert's bonus action doesn't really increase your effectiveness by very much. You want to use your bonus action in a game-changing way that lets you kill monsters you otherwise couldn't kill, and claim treasure you otherwise couldn't claim, and spend less resources than you would otherwise spend.
Increasing your attack rate DPR by 25-50% at the cost of your bonus action, a smaller damage die size (d6 instead of d8), a feat and having to be within 120' isn't a qualitative advantage. (At 1st-5th level when it increases it by 100%, Crossbow Expert is much more appealing, but not so at higher levels.)
Taking zero damage in a fight at the cost of a bonus action and either a spell (Expeditious Retreat) and open terrain, or a feat (Skulker) and two levels of Rogue is a qualitative advantage. Skulker + Rogue also increases your DPR by the way by giving you advantage on many of your attacks. Most monsters don't have blindsight or tremorsense, so you can hide from them behind obstacles or in the dark even if you're within darkvision range, because of Skulker. Skulker also ensures that you don't lose advantage unless you hit with your attack, which for someone like D'Artagnan the Swashbuckler (although he's not a Skulker until he levels up) means that his d8+4d6+15 (33) killer shot will almost always be made with advantage--it can't be blown on a miss.
Having a bunch of undead meat shields throwing nets to restrain your enemies and also tearing them apart and getting in their way at the cost of a bonus action and a spell or three (Animate Dead) is also a pretty good advantage.
Animate Object is similar to Animate Dead but less likely to be available to an archer, unless you think of a Sorlock as an archer. Also, Animate Dead has better action and concentration economies.
Hiding, mobility, and meat shields are the main good uses of bonus actions for archers. For other PCs, good bonus action uses can include wildshaping, teleportation, and maintaining damaging spells like Bigby's Hand.
Let me first address the first part, where I feel you bundle together different topics.
On one hand we're discussing how to best use your bonus action (I know full well you technically don't have it until you do, but still), on the other Crossbow Expert.
Had Crossbow Expert granted only the bonus attack, that would be one thing. Or if the bonus attack came from somewhere else and CE only provided other stuff. But that's not what is happening.
So you can't dismiss CE only on the grounds there are better things to do with your bonus attack. CE cruically removes the risks of entering melee. That it provides the equivalent of two-weapon fighting (the attack mode) as well as two-weapon fighting (the fighting style) is, well, not just "a bonus" since it's better than that, but still.
I can't say which of SS and CE is the most crucial part of enabling the ranged tactic.
What I can say, however, is that just focusing on the use of bonus action is comparing apples and oranges.
Yes, if you somehow could extract just the bonus action benefit, and replace it with something else, then you might want to consider that. But you can't.
So the bonus action from CE is much much more than DPR - it's the choice of the ranged tactic, which IMO is supreme. Talk about qualititive advantage!
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The other part requires us to theoretically assume you could just slot in skulker or whatever for the bonus action crossbow shot; without replacing anything else for our ranged build.
That said, a few questions:
"Taking zero damage in a fight at the cost of a bonus action" - could you be more specific, please. What exact build choice and what exact combat action are you talking about here?
As for Expeditious Retreat, yes, it's a bonus action spell. But you can't cast that every round (and you don't want to either). So it is by itself a poor substitute for a bonus attack. Or more to the point, nothing says you need to choose here. If you have this spell, you can cast it round one, but then make bonus attacks other rounds.
Skulker seems to me like a way to supply advantage, and as such, it's not a bad option. However I have several caveats: a method that grants advantage to everybody is far superior to one that just grants advantage to one hero. So skulker is on par with barbarian reckless attack in that regard, but worse than, say, Hold Person or Ki Stun.
Also: hiding (not the "getting advantage" part, but the "removing yourself as a target" part). I'm afraid hiding has a requirement, or it is fundamentally flawed. That requirement is - everybody needs to do it, or the monsters will simply hit someone else. And you don't want to make the monsters focus their fire any more than they already do.
(If your DM always plays monsters efficiently, this caveat disappears. But I would bet most DMs don't)
The basic building block, taking Rogue levels, on the other hand, is probably a truly strong tactic. The way Rogues get to use their bonus actions is certainly very useful, bordering on ridiculously strong if everyone in the party has a means to gain such high mobility.
"undead meat shields" - no objection there. When it comes to spamming conjurations it kind of breaks the game in its own way. I prefer to simply not to think about it, which doesn't mean your suggestion lacks validity. I wouldn't call that a tactic based on your bonus action as much as a tactic based on conjuration spells, though...
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In the end, though, it doesn't matter, since SS/CE isn't primarily about the bonus action, but about 120 ft "reach".
I would more consider your suggestion to take Rogue levels (for the hiding, mobility) as something you might consider adding on top of ranged, rather than replacing it.
