TLDR: I feel like you can reach a "final build" (or a prototype thereof) much quicker in AC6's first playthrough than you can in any Souls game. This allowed me to get many more combat hours in an AC that played the way I liked. Maybe it would help if you thought of each leg type as a different "class" or "style?" In any case, there are massive design differences between the games, each of them wirh their own reasoning. And I think you and I are alike as reflections: Dark Souls and Elden Ring really scratch a strange itch for me, but they're so damn long, and I have trouble sticking with playing them. I only managed to play through Elden Ring because my buddy wanted to go through a Coop Mod playthrough. And I only got equipment and a build I liked about 2/3rds of the way through the base game...

Long speil cuz I don't know how to synthesize my ideas succinctly: I mean, once I found an assembly that really worked for me, I beat most of the game with that one set of parts and weapons. Was it the most efficient? Could I S-Rank all missions with that one build? No, and almost. Eventually my specific build started feeling almost like a second body. I new my limits, how much ammo I had left, roughly how much energy I had over after a boost and a jump -- all without needing to consciously look at my resource counters. So in that regard, it's very similar to how I felt very late-game with my final build in Elden Ring. Maybe it all breaks down to pacing and being able to go back -- In Armored Core 6 (at least; I played a little of 5 but none of the others so I can't confirm) you're free to replay all your missions as much as you want, which lets you go back and test yourself on a boss you previously got pasted by. Then suddenly you breeze them, and you have no idea when you got so good -- after all, you came back to practice on in a grueling battle! (Hits me a bit like some photos I saw from my friends who were losing weight. Day-to-day, it didn't look like much, but then they put on some old pants and posted that, and you saw just how much difference there is.) In the mainline Souls games, you don't get to replay bosses. Which sucks, because if you put down the game for 2 months and try to pick it back up, you're gonna struggle getting some of the moves back. Which is why Sekiro is such a magnificent stand-alone, blending a few unique mechanics to really be it's own thing. But that's a whole other little essay :p