Predicting that Serena's at the edge of what she can accomplish is a bit like predicting that the Patriots will fall apart before the day that Belichick and Brady retire. Sure, it could happen, and one of these days it's going to happen - but the list of people who make that prediction only to be proven wrong has gotten quite long, and gets longer every year.
The first 3 names on your list are at the peak of their talents and would be a challenge even to Serena's A-game (I particularly fear Muguruza as a Serena fan, given their history at this tournament), and arguably Kerber as well, though she's a bit off-form. But if her trajectory we've seen thus far in this tournament continues, she'll be no more than a slight underdog against any of them.
I mean, we talk about her record against Sharapova, but her record against Wozniacki is 10-1 (last: 2014 WTA Finals SF), against Halep 8-1 (last: USO 2016 QF). Kerber has 2 wins against her including the AO 2016 final. Basically, if Serena's legs and endurance are anything like what they were even 18 months ago, she should still be a favorite, though not the massive one she was pre-pregnancy. I think it's fair to question her fitness, and acceleration, but her game is predicated on things that have little to do with fitness: serve quality, groundstroke power, aggressive shotmaking, and first-class footwork.
When you talk about "how she moves", you're combining a few things it's worth separating:
1. Recognition off the racket, decision speed on what to do (playing herself into form here)
2. Acceleration towards her spot (sub-par, we agree)
3. Top footspeed on longer runs (has never been a burner, but could do with probably losing 5-10 pounds to get back to ~2015-16)
4. Footwork for economy of movement (she's always been amazing at this, and still is - look at how rarely she's shuffling her feet still when going for big hits)
I think 2 and 3 are a combination of doing more cardio, which she famously hates doing in training, and natural body type and strategy (she's given up some of that to be more offensive-minded). But they can be covered up by being better at 1 and 4, and to exploit it you still need big powerful shots, which is how Muguruza beat her twice at RG, running her to places she couldn't get to. Far easier said than done.
I suppose we'll know a lot more after the Sharapova match, since she's playing like a top-10 player right now.