Interstingly, after leaving the project for a few days, things got even worse and now even a single panel is flickering. Once I've rebuilt it to the old config where 5V is shared through the 3-pin connector per column, it's still not gone, but noticably better. It gets worse with more panels though.

Unfortunately, I still don't see much of a difference, even after wiring the panels like you did (hopefully). Also using D1 (TX0 on my board?) for the first 3 panels, and D4 for the remaining 6. It still keeps flashing. See the attached wiring diagram, which I hopefully recreated somewhat correctly.

Oh, thank you very much! I think this will be quite helpful.

I've tried various things now, including rebuilding the entire panel assembly. And the flickering still remains.
iirc you've mentioned that you've desoldered some stuff, any details on that? Currently each bottom panel in a column receives 5V that is shared up through the 3-pin connectors

Yes, 6 panels in series, next to the remaining 3 in series. The ESP32 and the panels are connected to same ground on the power supply.

Well, no luck there. I've tried a bunch of different pins, with at least one panel always flickering. But once I remove the data output config for the pin handling 3 panels, it stops. You don't happen to have set up some special config for that, do you?

Hi, any update on the building instructions? I've managed to control my very own 3x3 panel, but I have some weird issues with unintentional flashing panels (or rather complete columns) every few seconds

Very nice, can't wait! Please let me know when that happens

This project looks super exciting! Are you going to post full instructions and part lists at some point?