"...a couple mics show slightly fluctuating signal in the led column..."
That's perfectly normal ...and I'd guess that the slight RF fluctuations for the two receivers in question are the only remaining effects of whatever RF was giving you huge grief with the previously inadequate rig ...and that the previous inadequacy probably arose from those whip antennae being attenuated by some physical aspect of (and/or metal-induced contribution from) their rigging.
"Does it make any meaningful difference which gain I choose?"
It certainly does.
Too little cable-compensating head amp gain is the easier to diagnose by dint of the low RF indications that show on receivers' front panels that often coincide with muting,
Too much will not only yield falsely high RF readings, (probably because it's "forcing" more undesirable 2.4GHz traffic down the cables, along with the targeted signals) but also generate muting that's then a lot harder to diagnose.
My rule of thumb for a given installation is to set and stick with the least head amp gain that's proven to be "safe" on a walk test in "battery save" mode ...and I suspect that the 3dB setting may be all you'll need.
As for then deciding the transmitters' power levels, ...if it's not practical to arrange a backstage "all mics on" pre-show test in the presence of a "sold out, standing room only" audience, I arrive at a "full power" or "battery save" decision based on another test with a single mic's transmission antenna being deliberately attenuated to "simulate" such problems as may later arrive.
If an assistant is available for the "walk", they provide the attenuation by deliberately "cupping" the antenna in their free hand, while orientating themselves at maximum range so that their entire body "blocks" the line of sight from transmitter to receiving antennae.
In the absence of an assistant, a reasonable "worst case simulation" can often be achieved by attenuating transmission through placement of the test transmitter inside a metal receptacle, such as a trash can or (moisture free) ice bucket that's located at maximum range.
Once you've optimised the rig we've been discussing, I'd now expect RF1 to yield a performance from your handhelds that will be rock solid in the direction you've aimed the P180s, seemingly all the way to the boundary of your suburb's zip code!