The actual problem in the question is that since there are only a couple of encounters per rest, mages can use all their tricks without conserving resources, and hence overshadow other characters.
Making long rests more costly is a part of a solution to treating this problem. There are others methods one can use in parallel.
I have been running a sandbox game for quite some time, most recently with D&D 5. Not quite West marches, but close enough, since the game involves lots of travelling through hostile territory with long rest pretty much impossible during the expedition, plus game master as a neutral referee. An extended description was treated as prescriptive, so I removed it from the answer to avoid misconceptions.
Resources determine how far you can go and how big a risk it is
Random encounters add time pressure and uncertainty. The first is not so important here.
I tend to roll three random encounter checks per 24 hours with 1/6 chance of an encounter each. This means that though most days are without an encounter, sometimes there might be three. If this uncertainty about how difficult it will be to return to civilization does not exist in your game, you might choose to adjust your random encounter procedure accordingly, or introduce one if you do not yet have such.
The mages should be somewhat reluctant to use all their resources so that they still have shots left for the random encounters, if they do happen.
Some adventure locations of yours are presumable more involved, living envinronments. After one has made one expedition there, the inhabitants will react, might escape, the place might be restocked, etc. As such, the players should have inherent encouragement to push as far as possible, which requires conserving their resources, typically.
But we don't have time
Make play in general and combats in particular go faster. There are all the usual tricks: roll attack and damage at the same time, become more fluent at using the digital platform of choice, keep play focused on playing, etc.
I will mention one special trick: quick resolution of combats. Since West marches is a challenge-oriented game, this is appropriate. The procedure goes as follows:
When anyone notices that a combat is not interesting (i.e. there are no non-trivial tactical or strategic issues at hand; the thing is resolved modulo dice rolling, essentially), they mention this.
Supposing nobody disagrees (the game master might know of some special powers of the creature, for example), someone, anyone, can suggest a quick resolution to the combat. This should be based on the assumed outcome of dicing it out. For example: "Everyone takes d6 damage and you win." or "The hydra gets two free attacks against everyone and then it is down." or "We use all our alchemist's flasks, more than 10 of them, but burn all the zombies." or "We spend 3d6 arrows and the slow beast is down."
Everyone needs to agree to the deal. In particular, if the game master suggested it, the players should agree, and the other way around. If not, clearly the outcome is in doubt; play it out.
This allows resolving boring combats quickly.
Also remember morale
Most creatures do not want to die and avoid situations where that might happen. Monsters and non-player characters should usually run or retreat when things get dangerous. If you want to formalize this, have them roll a wisdom save, or maybe have a player character who did something remarkable roll intimidation (or both). If the dice favour the enemy, they will fight on or attempt a gambit of some type. If not, they will retreat or run.
This should speed up combat significantly. Non-intelligent enemies often fight systematically, so even if they do not run, the combats can be resolved quickly, as per above.
It is a player skill and responsibility to act decisively
Players get to experience as much of the game as they manage during the session. If they proceed slowly in real time due to messy team dynamics, unclear leadership, poor rules competence, etc., then they miss out content in game. Having the players take responsibility of this leads to improved play on their part and more enjoyment for everyone. Furthermore, it teaches them relevant real life skills concerning team work and leadership.
Their characters can also learn more and find more treasures, or whatever their goals happen to be, if the players are fast and organized. They are less likely to die due to stupid decisions, confusion or hesitation.
Emphasize this point to the players. Competent play leads to better results both out of game and in game. D&D is a game of skill, when played in West marches style (and in a number of other styles, but not nearly all).