First, I'm pretty sure the game calls them "lesser" imps, not just imps.

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Second, enemies having special abilities isn't what makes a fight interesting. There's so much more that goes into it. What's not fun is a flat map where you hit me and then I hit you and then you hit me and then I hit you.

In the first fight, assuming you don't trigger a conflict with Us, the map has obstacles, high ground, and areas that are on fire. There's a sense of urgency as you're trying to make it to the helm, and you don't want to be stuck in hell on a burning ship that will surely crash.

Once you get to the helm, you experience the big fight. There's a time constraint. Get to the transponder, connect the nerves. You have a goal, and meanwhile, a powerful illithid is fighting a powerful cambion. Lesser imps are coming at you, blocking your path, along with a hellboar. Fight your way through.

These are interesting fights.

The phase spider fights are interesting, also. The location is part of what makes it interesting, not just the abilities of the spiders. You can go for the high ground, you can hide around corners, you can get caught in an effective ambush area. Maybe one of your characters tries to go high and gets stuck on a web bridge. Laezel can use her jump ability to maneuver around the map.

In my opinion, you're hyperfocusing on 5e stats, as if nothing can be good (or at least acceptable) unless it matches whatever the 5e monster manual says. But that's just not true. These things can be tweaked.

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Third, switching out party members based on how many short rest hit dice they have left isn't roleplaying. It's pure meta.

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Fourth, you mentioned all the issues with the tadpoles making you think the characters should push themselves to go as far as possible before resting.

So. Go as far as you can before resting. No one's stopping you from pushing yourself.

The notion that you have to rest after two to three encounters is patently false.

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Fifth and finally, you can't have it both ways. Either players can spam the long rest or they can't. If the long resting can be spammed, some players will and some players won't.

I don't buy into your theory on the psychological impact of charging food supplies for short rests.