Not sure if this is somewhat obvious to many sweep pickers but these are a couple of observations that have really helped in thinking about my sweep picking -
First thing I’ve noticed - a lot of great sweep pickers who change direction frequently during play have visible thumb movement that matches the duration of the sweep pattern, i.e. they seem to be including index finger movement in the escape motion/to control the speed of the sweep, similar to MP picking.
Observing Jason Becker - he does this very visibly. You can see it clearly in the Serrana arpeggios in the intro of the AIMM archive video here
You see that the thumb retracts slowly for the 5 string patterns and more quickly for the 3 string patterns, the MCP joint of the index finger is helping control the pick motion and integrating the escape motion into the motion of the entire sweep.
Why is this beneficial? Forearm rotation for escaping shorter arpeggios is inefficient relative to MCP joint movement, and personally I find it to be very challenging to incorporate forearm rotation rapidly for sweep patterns across fewer strings and it introduces more overall tension in my playing. I also find that using the index finger in addition to the wrist/whole arm motion gives more control over the speed/smoothness of the motion to allow for more even sounding arpeggios that feel more controlled and consistent.
A second thing I’ve noticed - with Jason Becker and Jeff Loomis, two great sweep/economy pickers with rapid arpeggio changes, they anchor with the pinky. Trying this out until I got used to it made it clear, if the guitar is not heavily slanted on my knee, anchoring the pinky really aids with counteracting gravity a bit and smoothing out sweep picking.
On the other hand, when I see great sweep pickers who don’t anchor such as Jason Richardson, they really are hiking the guitar way up on their knee which makes the whole arm part of the motion much more perpendicular to the ground - e.g.
When he actually plays difficult pieces live however, and has to stand up - you can see him clearly switch to pinky anchoring for string skipped/swept portions
Looking carefully at Richardson, it does look like his thumb bends/wiggles although it’s much less visible, but the way he learned sweeping was to force himself to alternate pick a variety of patterns up to 120 BPM 16th notes or faster and does quite a lot of crosspicked runs in his music which would also suggest MP picking.