Blue
Ravenous Bugblatter Beast of Traal
The balance now is longer fights (more rounds, not more IRL time) and more fights per day. Fighters keep on fighting. Casters run out of spells. If you run few or short combats, casters win.
This is absolutely true. Has three different resource recover mechanics - at-will, short-rest, and long-rest. It si balance around 6-8 meaningful encounters per long-rest, divided into thirds by short rests.
Harder but fewer encounters DO NOT balance this for casters. A buff that lasts all combat still lasts all combat, but now you only have 3 combats instead of 6. If there are more foes, area of effect can catch mroe of them. If there are more powerful enemies then debuffs are worth more.
Really, if you only plan on 3-4 combats a day, cut number of spell slots in half and your casters will be on-par. But sine the highest level slots are only 1 per, you still have issues in that you've giving the full amount or none. Maybe if you cast those the slot doesn't come back to the next day. But I don't recommend this - it will annoy your players, and no idea how it impacts half casters and multiclass casters.
I love 5e, but the resource management balance point having such a high number of meaningful encounters is a huge weakness.
Now, an alternate way to go is to steal form 13th Age, a d20 game that's similar to 5e. It decouples sleeping from resource recovery. In 13th Age it's balanced around four encounters, but that's an easy change to 5e's expectations. Have that short rest resources reset every other encounter (3 if one is minor), and long rest resources reset every six encounters. Suddenly resource management works. This doesn't work without buy-in from your players since it's a sacred cow.