Regardless of the setting, combat CR is only accurate if the group is having to fight "6 to 8 medium or hard encounters in a day." Mechanically, the term "day" should be considered the same as Long Rest. So if your group is doing 2 encounters, then heading home to sleep, you have a few choices.
1. Use the optional "Gritty Realism" rule in the DM Guide that makes short rests take 8 hours, and long rests take a week or so. This allows you to keep a slower combat pace between long rests, so that encounters are as challenging as they should be according to the CR. The downside to this is you'll possibly get pushback from players who don't like the idea of waiting a week to fully recharge their spells and abilities, no matter how much you try and explain that it makes no difference, mechanically.
2. Throw higher CR encounters their way. The problem with this method is that if you go just a few CR higher than you should, you start open the possibility of character deaths via one-shot crits and things, because the monsters are "balanced" with the assumption that players are higher level (thus have more health and defenses). The other downside is that those monsters themselves have more health, which means even if the offensive side of combat isn't too OP, it can still mean having several rounds where people are just hacking away at too many HP.
3. Balance encounters "as written" based on the normal CR calculations, but make a few key changes. When doing this, I generally drop the HD by a few points, increase the attack bonus by 1 or 2, and remove any special defenses that the party has no way to deal with (such as immune to normal damage, and the party is still level 3 or something). This is quick, can keep combat fast, and lets the monsters hit a bit more reliably, but their damage potential is still the same.
4. Use "legendary" templates for solo encounters. Lots of monsters in the MM have legendary abilities. The whole point of those rules is to address the imbalance of Action Economy where you have a group of people beating on a single monster. Legendary (or Lair) abilities essentially let the monster do more stuff in a round, and partly scales with the number of players. The abilities themselves don't have to actually sound "legendary," but they can be stuff like "make an extra attack" or "heal 1d8 wounds." Since they happen at the end of each player's turn, it can make combat feel much more like a good boss fight.
1. Use the optional "Gritty Realism" rule in the DM Guide that makes short rests take 8 hours, and long rests take a week or so. This allows you to keep a slower combat pace between long rests, so that encounters are as challenging as they should be according to the CR. The downside to this is you'll possibly get pushback from players who don't like the idea of waiting a week to fully recharge their spells and abilities, no matter how much you try and explain that it makes no difference, mechanically.
2. Throw higher CR encounters their way. The problem with this method is that if you go just a few CR higher than you should, you start open the possibility of character deaths via one-shot crits and things, because the monsters are "balanced" with the assumption that players are higher level (thus have more health and defenses). The other downside is that those monsters themselves have more health, which means even if the offensive side of combat isn't too OP, it can still mean having several rounds where people are just hacking away at too many HP.
3. Balance encounters "as written" based on the normal CR calculations, but make a few key changes. When doing this, I generally drop the HD by a few points, increase the attack bonus by 1 or 2, and remove any special defenses that the party has no way to deal with (such as immune to normal damage, and the party is still level 3 or something). This is quick, can keep combat fast, and lets the monsters hit a bit more reliably, but their damage potential is still the same.
4. Use "legendary" templates for solo encounters. Lots of monsters in the MM have legendary abilities. The whole point of those rules is to address the imbalance of Action Economy where you have a group of people beating on a single monster. Legendary (or Lair) abilities essentially let the monster do more stuff in a round, and partly scales with the number of players. The abilities themselves don't have to actually sound "legendary," but they can be stuff like "make an extra attack" or "heal 1d8 wounds." Since they happen at the end of each player's turn, it can make combat feel much more like a good boss fight.