In any game with feats, multiclassing, more than four PCs, experienced minmax players, and/or magic items, you need double-deadly or even deadlier encounters to challenge the party.
The DMG idea of an endless parade of weaksauce encounters where the outcome of the fight is never in question and no challenge is present is simply not what I want out of the game.
I'll be experimenting with some ideas to help with this concern, and a concern I have that forces combat at high levels to last several rounds and hours of real time. In my experience, I've run a variety of encounters per day ranging from 4 to 6. One of my players likes to try to get full utility our of his time limited spells since he is playing a warlock and has a few spells per day. He also never feels confident in taking a short rest in a dungeon, and my sorcerer player asks why she should take a short rest because she has very little benefit from it. I find that as levels increase and damage output increases, that HP values of the monsters increase. The net result is combat that seems to take a few hours real time. I believe the target number of rounds for combat should be between 3 and 5 rounds, based on what feels fun to play and what I seen in AL and on streaming games. Lately I'm in the 7 to 10 round range. To address this, I'm going to implement a bonus to hit and save difficulty and multiplier to damage starting at round 3. I think this is used in Savage Worlds and I heard Adam Koebel, but I'm not 100% sure. I need to iron out the details with my players, but the current thought is to add a bonus to hit and to save difficulty equal to the round number starting at round 3 (i.e. +3 bonus to hit or save difficulty on round 3, +4 to hit or save difficulty on round 4, etc.). In addition, a damage multiplier equal to the round number will be multiplied to the static bonus for damage. For example a weapon attack that does 1d6+2 damage would then do 1d6+6 damage on round 3. A spell attack would receive the benefit of adding 3 for each die rolled on round 3, thus a fireball is 6d6+18 or a magic missile is 1d4+4 per missile On round 4 the multiplier becomes 4 and all damage is adjusted accordingly. I feel like this will end combat quickly and possibly still cause a resource drain requiring the players to think about moving ahead.
In addition, I'm considering adding a benefit to the players if they continue on for the day without taking a long rest. This is something I heard Matt Colville mention and it's from a Warhammer game. Once the character reach 4 rounds of combat during the adventuring day they each receive inspiration. Once they reach 7 rounds of combat during the adventuring day one of the PCs get the ability to inspire ally as a bonus action granting some temporary HP. Once they reach 10 rounds of combat in a day they each get 1 lucky point to use. Once they reach 13 rounds of combat they've worked together so well that day that one PC gets to switch initiative order with another with another PC once in the encounter. Again, I need to iron this out with my players.