Things just seem so "messy" right now - which is kinda typical during an edition change. There are so many variants of "D&D" (not to mention countless other systems): 2014, 2024, Level Up, Tales of the Valiant - and I'm sure there are others.
I need more tools than what's in 2014. I'd like better monster design. I'm aware that comes with some degree of added complexity, but hopefully I can find the right ratio between fun and complexity for our group.
Currently, I'm needing to recharge and going to less complex systems (leaning to Dragonbane for a short campaign).
As far as monster design is concerned... yeah, pretty much any of the non-WotC monster books are more "complex" because they give monsters more options in their statblock for things to do. The more things they can do, the more you as a DM have to remember to use during a fight... but at the same time the more varied the results of fights will be.
The 5E14 MM is in a lot of ways like AD&D monsters... where for a lot of monsters there's nothing more than a melee attack, ranged attack, and maybe a single feature (for defense or added offense or whatever.) The difference though is that monster HP was considerably lower in AD&D, which meant monsters died faster. But that was okay back in AD&D because most PCs (through their classes) didn't have much of anything advanced to do with actions either-- they had pretty much just a melee and ranged attack as well.
The results of which meant that combats had little in the way of "interesting" results... but they all finished so quickly that it didn't matter-- the party would just move on to the next part of the adventure (a lot of which was exploration and did not involve combat, which meant their lack of interesting combat abilities wasn't an issue.)
But in these later editions of D&D... we've wanted to give PCs more and more "interesting" actions to take and ways to build your characters, so on and so forth. And that results in more things for the PCs to do, but also a lot more time needed for them to do it. As well as needing more and more things for the monsters to do in combat to match.
Which is all to say that it makes all the sense to me in the world why so many people DO migrate to older games (or OSR remakes)... because they often make combat shorter and less involved by comparison. And as a result, players and the DM are forced to make the narrative / non-mechanical parts of the adventures (exploration, socialization etc.) more important. Thus covering over the fact that combat is over so quick and doesn't have that those "action movie set pieces" we've all come to expect in modern D&D nowadays. But if that's better for you as a DM-- more focus on verbal exploration and socialization with players telling you what they wish to do, and you telling them back what happens and what they find-- rather than strictly mechanical dice-rolling that you have to remember all the rules for... then going more old school certainly makes the DMs job easier. Other than having to think up and create all that exploration and socialization for the adventures of course, LOL!