Got it done today. It took a lot of zip ties. Taking the old 6 circuit transfer switch out revealed some weird stuff.
Transfer "a" and "b" were the X and Y from a 4 conductor wire, going to circuits 13 and 15. Seemed odd to me to have one romex bundle feeding two circuits. It is a house from the 80s and it's white romex, so back then from what I've been told outlets and lights were on same circuits. Maybe they ran red to one room and black to another to save on wire costs and shared ground/neutral?
Transfer "c" was one hot wire from another 4 conductor bundle also split between breakers. it fed breaker 17.
Transfer "D" went nowhere, literally. the red wire from the switch went into breaker 19 but the black was tied off to a random wire nut. So when I removed it, 19 was left empty.
Transfer E and F both were one wire from two separate 4 conductors again. So basically these were all on old lines and i thought it weird they were sharing.
Anyways, put everything back to the breakers (except 19) and tested things. Once that was cleared, I shut power off again, tidied unnecessarily long wires, tied bundles, installed my interlock switch. I had to grind off a bit to prevent the breakers on the other side from hitting it which prevented them from fully engaging. Then I installed the Emporia Vue smart home monitor. The clamp ons were easy, but I could not for the life of me cram everything in the panel like the instructions suggest so I knocked out a few holes and ran the wires outside to the module. It's a temporary fix until I can figure out what to do, but now I can watch trends and see circuit power then decide how I want to do things with a generator.