Those elements certainly lift the quality significantly, and rewards super fans!
Fujimoto didn't draw Denji since the first part ended in december 2020. So when his first appearance was recently, I expect he was eager to draw him again.
Part 2 isn't revolving around Denji, his saga was part 1, and we really have nothing left from him besides his standard Devil-hunting. Mini-Makima has no relevance right now, though I'm sure she'll show up later. Things need time, and right now just isn't the time to get more Denji. Especially if you read chapter 98 then 99 back to back, get into the mindset of someone binging this manga compared to waiting for part 2 for a year and a half, it's a natural progression. Denji's saga was complete, his character established and grown, and now he reigns as the strongest Devil we know. There's really no point focusing on him. So, we've got Asa. Not only does this significantly expand the world building, but we get another characters views on the world. A view that's so much different from denji's, yet all familiar due to the fact they both harbour powerful Devils as ressurected corpses.

While yes it's true Fujimoto has done a few one-shots in the time since part 1, I doubt he'd let his most successful manga down by forgetting the plot. No, he's probably been planning this ever since part 1 ended. Considering how part 1 felt like a definitive end to the manga, it just makes sense to change the protagonist to someone who's not had their character arc realised. Though of course we do get bits of the titular Chainsaw man sprinkled here and there as we see his coping of the previous events, while also for the very first time experiencing School as well as his usual Devil-hunting. Not sure if he's also being a guardian of Mini-Makima or he's letting someone else take care of her, but the control devil isn't a problem anymore, and it's safe to assume she has no more relevance on the plot at the moment.

Reze's arc ended when she was consumed by Makima, she has no bearing on the plot. Power 'lives on' through Denji by her blood being inside him, that's all we know. I doubt she'll be brought back because that will rid the significance of "The door" sequence. But she's not exactly dead, her Devil (Soul) lives in Denji.