This answer is based on transitioning from another module (Dragonheist) to OoTA
You don't have to depower them, nor cherry pick the equipment
They'll be naturally weakened without their gear
A fifth level party without any gear, in shackles (and if your party is particularly powerful, in separate cells) is already depowered enough. You still have in your hands a very similar situation as if the party were level 1. Sure, the fighter has 2 attacks - with his fist in shackles. The wizard/cleric has 3rd level spells - without their material component/holy symbol.
Sure, they might get some cool extra stuff they can do, but this will add to the possibilities and plans they can came up with without diminishing the danger of the situation.
They can get their stuff back
Instead of cherry picking the equipment they get (why would the drow let the wizard keep his spellbook?), remember that they can get their stuff back. The increased possibilities they get at fifth level is balanced by the fact that besides escaping, they have to get their stuff back. This is not so simple as escaping anymore, unless they're willing to part with their gear.
Now you have a classic trope! How many good stories have a part where the protagonist barely escapes the clutches of the bad guy and now has to stealth trough the lair to get their stuff back?
While you totally can use this situation to remove some of the most powerful gear, you don't need to. For example, the enchanted sword catches the eye of one of the elite warriors that has left the outpost already, the spellbook did the same for a drow wizard but he's still assigned here: boom, logical reason to keep some powerful gear and remove others.
Small climax before the dangerous underdark
Getting their stuff back can even work as some kind of "last moment of badassery". Once they get their stuff back, they might be tempted to show those pesky Quaggoth who's boss. Let them
That moment when they feel powerful and badasses will be great. This will then add to the desperation the Underdark is supposed to give them when all those mighty fireballs barely slow down the relentless drow's hunting parties. The high level priestess and the Underdark's deadly environment, and several far more dangerous creatures that roam the underdark challenging the party adds to this feeling of dread.
Another idea from experience as a player
Depending on what danger you want to focus on (environment vs enemies), you can have them beat by Exhaustion. Throw some enemies at them, barely a few short rests as they entered through a particularly active part of the underdark, then get them have some environmental problems, so that when the drow ambushes them they will be exhausted, without their big guns having been beaten up by the environment.
What my DM did was get us into the Underdark, be attacked by some kua-toa, and when we found a river it led us straight to a waterfall after which the drows captured us.
It added to the feeling that we don't know this place, we're in constant danger and that we need to be careful for every step lest we end up in another waterfall or worse.