In Marlin, along with the standard way of retraction, there's a feature that will retract all materials instead of just the active one. I want to try this out in the future since, to depressurise the heatblock, each channel will only have to retract 1/8th as much as a single channel. For example, in perfect conditions, retracting 8mm from one channel would be equivalent to retracting 1mm from all 8.

However, reality is likely different. The length between the bowden tube means that the tip of the filament might not even move until the extruder retracts 2mm, for instance. For TPU, this is likely more.

Perhaps the firmware retract feature would be a good thing to look into. I've never used it before, most likely because Cura doesn't natively support it, and it just seems that it merely moves the already-basic slicer settings into the firmware without provisions for multi material situations.

Another thing to consider is Linear Advance. I haven't gotten around to testing it out, but I assume that it only works on the active channel. I'm wondering if the other channels could be used to assist in some way.

Ideally, there'd be a per-extruder toggle whereby some won't retract and others will instead, to make up for it (e.g. retracting all but TPU channels). This should mean that both retractions and LA can react faster to flow rate changes.

The main edge case I need to worry about is one or more channels slowly ejecting themselves, i.e. a few retractions or LA corners later, the filament in a channel slowly moves backwards, -0.3mm here, -0.9mm there, and accumulates to -30mm.