Making long rests more costly is one way of treating this problem. There are others.

Long rests happen between adventures/sessions, not during them

I have been running a sandbox game for quite some time, most recently with D&D 5. Not quite West marches, but close enough. The purpose of this section is to explain how my games have similar dynamics. For solutions, see further below.

Especially with a rotating the cast, the simplest way to go is to simply say that a long rest happens between adventures/sessions. I have a little bit more complicated rules, where a night's sleep gives back one hit die and the use of one ability that normally recharges upon a long rest (details here).

To make this systematic, the rule for long rests I use is that you need to be relaxed and not on guard for 24 hours to get the effects of a long rest. Sometimes this can happen during an adventure, but typically it is a pretty big risk to take to just go ahead and relax in the wilderness, not to speak of an adventure location.

But, aside some unusual corner cases with the most involved players, you simply have the long rest between adventures/sessions, and that is that.

Keep track of in-fiction time with care

Assume you want long rests to take, say, a week. You can combine this with a West marches game by keeping track of when the characters are rested. If the player wants to use that character before then, they will not have rested.

This approach works best with character entourage/stable, where each player has several characters, only one of whom are active during a particular sessions. If one character is resting, they can play another.

Note that some players are not interested in this at all, whereas others are fine with this. Talk to the player base before implementing.

As an added bonus, if you run a high-lethality game, players tend to take character loss better when they have others in play, and they are also likely to be more prepared for the eventual character loss.

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.

I have been running a sandbox game for quite some time, most recently with D&D 5. Not quite West marches, but close enough.

I have been running a sandbox game for quite some time, most recently with D&D 5. Not quite West marches, but close enough. The purpose of this section is to explain how my games have similar dynamics. For solutions, see further below.

But, aside some unusual corner cases with the most involved players, you simply have the long rest between adventures/sessions, and that is that.

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 one way of treating this problem. There are others.

But, aside some unusual corner cases with the most involved players, you simply have the long rest between adventures/sessions, and that is that.

Keep track of in-fiction time with care

Assume you want long rests to take, say, a week. You can combine this with a West marches game by keeping track of when the characters are rested. If the player wants to use that character before then, they will not have rested.

This approach works best with character entourage/stable, where each player has several characters, only one of whom are active during a particular sessions. If one character is resting, they can play another.

Note that some players are not interested in this at all, whereas others are fine with this. Talk to the player base before implementing.

As an added bonus, if you run a high-lethality game, players tend to take character loss better when they have others in play, and they are also likely to be more prepared for the eventual character loss.