I guess a point to consider is that with Tracker Plus you have 16 voice polyphony (additional 8 voices shared by the 3 synth tracks). When I was using the Tracker OG I found the 8 tracks sometimes limiting but since switching to the Plus I have never found it an issue as I use the synth tracks for the melodic elements. I have always assumed the 8 audio track is a hardware limit of the OG and that this was one reason the Plus was developed, so in some ways the answer to this is to upgrade to the plus? Personally I think the approach of the Plus (synth tracks triggered by MIDI tracks) works better rather than subtracks as it preserves the simplicity of the workflow.
I guess likely considered already but could also consider that another workaround is that if you are building a drum track from hits rather than loops you can get lots of different voices into 1 track as long as the samples are short and don’t overlap.