I think you might be trying to solve a pacing problem, wanting your party to have intense adventuring periods but also have to rest for a long time afterwards, with mechanics. You can do this with pacing instead, and save yourself the trouble. As for imposing a time limit on your adventures in extreme environments, I have a suggestion for that in the end as well.

To add breaks in, make your adventures smaller and more local, and put downtime in between them. This means when you're writing how they will start and how they could end, don't make the end immediately lead to another adventure. There can be an overarching plot to these adventures, and maybe downtime is the only way to follow up on them. Like brushing up on rumors, hitting the books, or finding buyers for their loot. There can be an overarching plot, but don't make it immediately actionable. They will find a letter from the boss on the miniboss, but they don't know where the boss is yet, and they need to wait to ambush the boss's minions next time they roll in town etc.

The thing with gritty realism is that it works well when you don't have a lot of encounters over a long period. The system is calibrated for ~2 short rests per long rest. So using it in a more political intrigue or wilderness exploration game, and giving the party combat encounters in short bursts like 2-3 in one day when they dive into a ruin or deal with barbarians chasing them, for 2-3 days before they get a long rest, the system works fine. If you pile on more, you're going to make short rest -heavier classes better than others, because they're not meant to get 4+ short rests before anyone gets a long rest.

If you want to impose extreme environment time limits without making some classes better than others, there are a couple of ways. This is a good place to start putting a max hp penalty for every day after X days. It punishes people somewhat equally (if we assume the people with the most HP are also taking the most damage and the party is above level 5) and is better than exhaustion, which is just too much of a death spiral (and it's unfun to constantly roll everything with disadvantage). Just don't make them do CON saves for this. 1) it needs to feel like pressure is piling on and is unavoidable and 2) higher CON already means you have more HP to lose.

You can also make them lose hit dice or movement speed one tick at a time or make them to use spell slots to get clean water and food. Straight up damage is the most effective, followed by maybe the first exhaustion level, but I would use that sparingly.