Encouraging them to have the rogue be sneaky doesn't necessarily require discouraging them from using the druid. Where possible, encourage them to work together. The druid can scout ahead to give the rogue information about the layout and the guards, create distractions, or keep watch while the rogue picks locks, dispatches guards, and the like. Each can handle the aspects of the challenge better suited to them, or give bonuses to their compatriot.
This has more players involved and opens up more interesting strategies.
The Vlad Taltos novels offer some good ideas. Vlad is an assassin (at least initially) but also a witch (Gender neutral term in the setting) with a familiar (Loiosh) who is essentially a tiny wyvern with an ecological niche very roughly in the vicinity of raven/raccoon/vulture/rat and they are telepathically linked. Loiosh is sometimes a distraction, often a scout, and always sarcastic.
Heist movies (Ocean's Eleven, etc) also give some good ideas for how to run co-op sneak this, as well as maybe some ways other part members could get in on it.
If you were running GURPS I would recommend making heavy use of Complementary Skills (Make a skill roll which then based on success gives a small modifier to another skill, possibly made by another player) For instance one player rolls Fast-Talk to distract a guard which gives a bonus (Or penalty on crit failure) to another player using Stealth. I'm not sure how adaptable this would be to D&D though.
While not a RAW answer, I would consider how the pearl of power works and has been errata'd over time in this scenario.
The original text of the item read:
You can use an action to speak this pearl's command word and regain one expended spell slot of up to 3rd level.
Over time, Crawford suggested that warlocks should be allowed to regain slots with a pearl of power, though with the phrasing being a personal ruling moreso than an official one:
I'd allow a pearl of power to take an expended spell slot of 4th level or higher and turn it into a 3rd-level slot.
This was eventually errata'd into the item itself, whose text now begins with:
While this pearl is on your person, you can use an action to speak its command word and regain one expended spell slot. If the expended slot was of 4th level or higher, the new slot is 3rd level.
While it's certainly possible that the lack of such text in the new barbarian ability may have been intentional, historically it seems they haven't had a problem with restoring warlock spell slots at a lower level when such a feature is available.
Additionally, I would consider the transition from party level 6 to 7 in the scenario where warlocks are the primary/only spellcaster in a party with a Wild Magic barbarian; that part of the feature would effectively suddenly "turn off" at level 7 if restoring lower-level slots is not permitted, which is quite awkward.
Best Answer
The d3 has been used for many different mechanics.
This search on DnDBeyond gives all the instances of "d3" appearing in the DDB compendium material, which covers the first three sections here. Note, this one has several false positives, as "D3" is used to denote locations in many adventures. All the other searches I link here use "1d3" to get hits.
The d3 first shows up in the introduction to the Player's Handbook, where it is explained how you can simulate one with a d6:
We now examine many of the ways the d3 has been used.
Falling down a chimney.
In one of the adventures published in Explorer's Guide to Wildemount, you can fall down a chimney:
Betting on dinosaur racing and meeting with princes.
In Tomb of Annihilation, you can bet on dinosaur races at one of the major towns. Two of the possible results depend on the outcome of a d3 roll.
In this same town, the party can seek an audience with a merchant prince. The module states:
Having close family relationships.
In the character building options in Explorer's Guide to Wildemount, you roll a d3 to determine how many of your family members you are close to:
And a plethora of magic items.
This search shows 33 magic items that use a d3 in some manner, most of them are for recharging at dawn. I will not list them all here.
The Ioun Stones use a d3 to determine how far from your head the stone orbits.
The decks of Many Things and Several Things use a d3 to determine the number of times a certain ability can be used.
And a few monsters.
This search gets us the mosnters. Five monsters use a d3 to determine the number of creatures they summon with a particular ability. For example, the Crokek'toeck:
The Assassin Bug lays eggs inside you:
The Olhydra makes it rain:
How many slaves does a Booyahg Goblin have?
This search gets us the goblin playable race. But this is misleading. DDB took the Monster Lore section on Goblins from Volo's Guide to Monsters and put it on the same page with the goblin playable race information. I recommend against giving your players slaves.
And an indeterminate amount of times for random encounters.
This search gives an overflow of results at DnDBeyond. I'm not going to count them. Most are random encounters from Xanathar's Guide to Everything, but there was a hit for Tales from the Yawning Portal, as well as one for Icewind Dale: Rime of the Frostmaiden. .