[Synth Reflex] makes triggering [Regulators](4) quicker and reloads your weapons while you're using them.
Mesa's Waltz is just too slow to be effective. If you want to move with your Regulators just bullet-jump and pull them out mid-jump.

Range affects the minimum Peacemaker area, making it significantly more usable when fully ramped(minimum area), but only [Stretch] really fits without sacrificing other important stats. But Peacemakers are quite usable even at minimum range with lots of re-casting, so Stretch isn't an important mod; a flex slot.
Good mods for the flex slot:
Stretch - Makes Peacemaker targeting area larger and Shooting Gallery(2) disarm enemies from further away.
Adaptation - Great for lazy play. Can even be used unranked to save capacity, without losing much effect.
Transient Fortitude / Augur Secrets - For a small increase to Peacemaker damage.

Regulators build: https://overframe.gg/build/27826