Players don't need to complete exploration of the city at all. After a couple of days bossmonster turns up and takes down the barriers herself.
Not a couple of days. 12 hours for Avarice and 24 hours for Auriel. Or 1 long rest. And when they do show up, hunting parties get sent out and you get interrupted nearly every hour, making long rests impossible after that. So yeah, you pretty much have to explore the city (or get exceptionally lucky to explore the right areas) all in the first long rest. Otherwise every battle after that takes resources and you don't get them back. Which is important because being at 1/3 hit points and ability/spell slots makes a big difference when you run into Auriel. And even before she does show up, you have a 50% chance of an encounter every hour or each time you explore an area, whatever happens first
But my party had no trouble resting using Rope Trick and/or Tiny Hut (which can be cast by an NPC ally if the party don't have it).
Again, "well, in order to take a rest, you must have this NPC with you or a PC that happens to have that spell in their repertoire, prepared, and slots to cast it. That's hardly a given. Also, LTH isn't invisible. Take your rest, then when the dome goes down you're surrounded by a ton of enemies just waiting for you.
And woe be to the player who touched the giant spear and is now too large to fit in the hut to begin with.
Sure, but then you have to reenter and go through the gauntlet again, not able to take any rests because you're getting attacked every hourThere is also nothing to stop the party leaving the city and resting outside it.
A single target ability. It's no big deal if one character spends a couple of rounds on the bench. It only took two rounds for the rest of the party to destroy the second form.
I don't think you read what I said. It's not a couple of rounds. Once you're frozen, unless you have prof in CHA saving throws, you're pretty much there to the end, taking damage each round. And it doesn't go away when her 2nd form does.
Who goes into an adventure called Rime of the Frostmaiden and doesn't prep fire spells? Who can't figure out to use fire to melt ice?
For one, the crystal isn't ice, it was a crystal. Secondly, the PCs don't know it's immune to everything except fire, so it's likely they would just attack it with weapons or other magic first. Thirdly, Auriel would target the PCs who could use fire or who did the most damage since she's not an idiot, right? Which incidentally is what happened to us. Fire druid got frozen, and rune knight got frozen. Since both of us had 10 CHA, there was no way we would ever succeed in being free. Leaving the monk, rogue, and warlock to face her and her mephits alone. It's not likely that they could take her down in a 2 rounds.