Start doing planning sessions
Disclaimer: I haven’t tried this for your exact situation, but have tried several elements of it individually to address players interested in heavy roleplaying with planning involved.
This does require buy-in from your players to keep themselves in-character. If a player rolls like garbage during planning, that crappy roll needs to be followed in-character. If your table can’t do this, I wouldn’t recommend this technique.
The Shadowrun style of play would strongly advocate support of this idea by the players, mostly because combat can be so much more deadly in that system. Given it’s heavy focus upon breaking and entering into places you’re not supposed to be, it provides heavy support for things like magical scouting, hacking the information systems, etc. Just about everything short of going in person, but you might even send the party face in disguise to get a rough idea of the general security protocols. As a result, the party can easily spend several hours just making rolls and roleplaying associated with the scouting itself.
So to address your original concern, my recommendation would be to try and take a page from SR and lean into the problem. Make your overall dungeon’s combat encounters notably harder, but allow the party to benefit from as much foreknowledge as they can get via their scouting efforts. Give them information on terrain, traps that are located (and can be used against the inhabitants), give them numbers (give them numbers that strongly suggest direct assault isn’t a good idea), identify doors (including which ones can be blocked/barred), give them enemy physical descriptions (and let them make knowledge rolls to identify abilities). Let them build a map, let them fill in the information they think is important, let them plan and scheme different ways to engage with each room as they move towards their goal. I wouldn’t recommend giving them the location of any secret doors unless the Imp’s Devil Sight would be the reason they’re spotted; Imps don’t have particularly good Perception scores, so leave finding those to the party itself (they may have a good idea where to look based on the map they’ve made).
It will be your choice as to whether you want to give them time for things like Long Rests to address the problems they’ve identified, but know that if you do you’ll definitely see much more efficacy from your prepared casters.
Once the planning is complete, it’s up to the party to actually put their plan into action. And if it goes at all like SR, 50% of things will go according to plan, and the other 50% is going to be a crapshoot because of bad rolls, which is perfectly fine. There’s a lot of fun to be had because the party’s fighting a troll which they thought was an ogre because some rolled a 2 on a knowledge check and they don’t have an obvious source of fire.