I’m still waiting on shipping for a few things, but this capacitance of the wire thing rang true for me. I redid everything with 22awg stranded wire, which I had on hand and which is the size the original wiring). I had been using 20awg. This actually helped a lot. Now all six LED strips (2 original and 4 new ones) are acting properly except one of the original strips is flickering a bit. It’s following the data line and doing what it’s supposed to do! But it’s flickering a little. Big improvement.
I don’t really understand how to use a 74HCT04 in this setup. I know this is a dumb question for people who understand these things much better than do I, but is there anything else that I could use to condition the data lines to keep their shape properly? Something that can be put in line with the wiring to help me keep this simple and (relatively) easy?