@darkcruix when in doubt you want "star ground" - i.e. everything having it's own ground wire converging on one point. This should not matter for this project though.
Please double-check you connected ground on BOTH sides of the PCB - on newer beamsprings there are no vias so PCB grounds are separate on each side, and having a floating trace which weaves around all the keys can cause all sorts of trouble, especially in beamspring (because it's flippers are down at rest, allowing the signal thru - unlike model F).
I can't really check the actual mapping you have from the picture you posted - it looks nice, but as you probably noticed when soldering, 30 parallel long wires of the same color are not easy to trace. It looks close to what it should be, but..
Also please define your matrix for the firmware with EXACTLY THE SAME DIMENSIONS as your physical matrix. For model F it doesn't really matter, but for beamspring all unconnected columns will appear pressed all the time, tripping the safety checks on startup.
a) use logic please, and take time to actually read the warnings. The "project" column is right there, not even out of the viewport. It tells you it is not one of the projects you're trying to build. What is the logical conclusion here? It is not relevant.
b) I'll remove ALL the default settings and configs of ALL KINDS next time I'll be committing things.
The README describes how to do it without any defaults. Just do it in steps. Planning => Soldering => Thresholds (+hardware settings if you can't get the signal/can't get stable readings) => Layout. Make sure step X is done right before you move to step X+1. Don't cut corners - this is a minefield, not a park.
This is not xwhatsit which has custom hardware - this is a Lego set of standard bricks. You can build, like, WORLDS from those - but it requires imagination, skill, and some thinking. This is both blessing and a curse. Especially thinking. I hate thinking.
If there will ever be a custom-hardware-controller (well, there's CSSK, but I don't expect more than 2 of those to ever exist, one in acrylic and one in aluminium) - it will be a different story. It will come preprogrammed, with matrix dimensions set according to keyboard it's made for, so all those problems just won't be there. But it will be limited to the keyboard it's made for.
Also come on, if you programmed 24x4 configuration and connected to the actual 23x4 beamspring keyboard, column 24 should be lit up no matter what you do. Is it not enough of a hint that you should configure it as 23 columns?