The combat isn’t less deep in DMC1. What’s less deep is the amount of actions and the combo systems. Combat isn’t not just combos and character mechanics, if it was, then any action game without combos would have “not deep combat” which isn’t true. Combat is a two way street, it’s your characters abilities vs the enemies abilities. It’s a matter of toolsets. Even in fighting games, there are lots of attacks a character has that actually have no value in neutral, and are entirely just combo fodder. When doing combos they are valuable, but in neutral, where the actual combat phase is happening, they have no use. DMC3 is very much like that where it has a lot of moves that are just not useful in neutral but are great in combos. It has moves that are the reverse, and moves that are both. In DMC1 there are very few moves that are for combo maximization compared to how many are useful tools designed to deal with specific combat situations.
As for the enemy design in DMC3, since you can have any mix of weapons with you, the enemies have to be designed to be defeatable no matter what weapons you have. in other words, no matter what tools you use, they should be killable. This means you aren’t really learning a good strategy to beat the enemy, you’re just using whatever you like to beat them since they won’t flip out of combos, or block certain types of attacks(vs being vulnerable to others) etc. Now, because DMC3 is designed with player expression in mind, it’s fine, it works, it’s just not very compelling combat interaction. It’s a ton of fun to play as the player character, but the sheer range of tools necessitates enemy design that is straightforward. This is why everyone hates Blitz’s in DMC4. That kind of enemy(Fausts too) was designed around Nero’s toolset which is way more focused(though even for Nero Blitz design fails IMO). Whereas for Dante to deal with them there’s not really any great answer. Anything kind of works but not amazingly well, or you can just learn distorted Real Impact which just kills everything, but no matter which you are going with you’re not really equipped to deal with a Blitz. You have to do really high level stuff to make short work of Angelo’s, Faust’s, Frosts, etc. with Dante, whereas even a mid-level player with Nero has options to deal with everything they have that are not esoteric high level stuff.
This isn’t just an enemy count issue, and the main reason enemy count in DMC3 went up is because those enemies are design to be killable in any way you please since the design goal is to just let the player do anything they want, rather than to design interesting situations. If the enemies were made in a more interesting way they COULD reduce the enemy count and it would be fine. Granted, the entire game design would have to change at that point and I don’t expect DMC to ever go back to the “actual” combat focused design as it left that behind a long time ago now.
I’m pretty sure Itsuno himself has said that the design philosophy for his DMCs is mechanics based, while Kamiya’s DMC1 and plenty of his other games too, have been, IIRC it was called “situation based.” Designing enemies around creating situations makes for more interesting neutral combat, while the former makes for more interesting punishment combat, to borrow FG terms again.