I know the following is not about panning each percussion module, only the last option is about this I think (hexmix).
But you could use the stereo mixers to the Rosie Return stereo set.
Problem is that there are not many input channels in most mixers.
BTW dot / ll8 and Burst seems like nice combinations.
Already thought about the Befaco - Percall (on my wish list)?
MylarMelodies has a 'suggested systems' techno machine item on YT.
There he is using the Befaco Percall partly as mixer.
I am also changing very slowly evolving to a percussion rack, but my budget has been slashed this year (because I am in between jobs now).
My 'optimal' stereo set would be 4 Percalls to a Doepfer A138S stereo mixer, but that is very expensive.
(Title: How to make a TINY live techno modular synth in only 62HP.)
Another option (see my "end game" rack) :
Rosie + pico DSP
combined with 1 or more of the following:
Axys (on my wish list)
X-Pan (on my wish list)
A-138S
I have a A-138N for each (Rosie) channel.
and a A-138S between the Pico DSP and the Rosie (stereo return input).
But I do not know if this would be possible with the hex modules.
another option :
MixZ tiptop (combined with TipTop drum modules) (on my wish list)
There it is possible to bus audio, instead of the need for cables.
Or going to 'real mixers' like HexMix / Hexpander / Hex VCA combinations. (also on my wish list)
like you said yourself.