So, as someone whose first 50 on HC was an archery/ninjutsu sentinel, I think it's broadly fun to have the opportunities, it's neat to be able to trade off offense vs defense, and it's nice to have a single-tough-target killing niche. I agree that feast-or-famine is kinda a weird theme, that the AT is slightly under-tuned, and it's frustrating to have the whole first-two-attacks thing when you are so likely to want to otherwise skip one of those attacks.
This is probably crazy, but:
What if the two opportunities became two mutually-exclusive toggles available at L1, not slottable. Offensive mode and defensive mode. Offensive adds a damage proc to all attacks, defensive adds a heal and endurance heal to all attacks. They might cost endurance or might not, might have a cooldown like Hybrid abilities or might not, but the point is that they're available for extended times (not 15 seconds), on demand, with no attack reliance. They give you some more durability and longevity when you need it, or offensive punch when you need it.
Then, replace the current -5% resists and -20% resist on opportunity hits with a -X% resist-all on all sentinel attacks, where X is the animation time, in seconds, stacking, with a loooong duration, like 30 seconds? Even 40 seconds?
No damage scale change, no target caps change.
So what's the upshot? Sentinels do what they do today, but a little better. They don't get much -resist on fast-dying foes (but that was never that big a deal), but in AV or GM fights, they eventually get you to -30%+ resist-all for the entire rest of the duration of the fight, making them really powerful hard-target killers -- eventually. It's synergistic with their set (you need to survive long enough to build up your damage bonus, hence the armor set) and the rest of their inherent (defensive toggle gets you where you need to go).
They'd also continue their role as "an easily recommendable newbie AT." You tell people to keep their defensive toggle on as much as possible and then just go shoot shit. Done.
This I think runs the risk of overtuning them, but they would remain outright bad farmers, I think, and less effective at clearing normal spawns that scrappers even with their offensive toggle.