yep you got it, just a single conneciton ebtween the V- on the power supply is enough to sort the data issues in most cases, just dont mix up Ground and V- as technically they are 2 different things (for some reason lots of people in this hobby use V- and ground as interchangable, its fine when...
proberbly not your issue but doesnt SFTP / FTP need to be grounded at each end for the shielding to work correctly?
I have similar flickering issues when the negative from the pixel string does not have sufficient enough current path back to the signal source
if you can SSH to the Pi and just run sudo halt that will shut the thing down and drop the SSH connection, you could automate it with scripts or create a one click action in something like home assistaint or FPP Script to do it based on user input or external button somewhere
the problem is not so much the latency as you can account for that its the variable latency that you cant account for - every single model of phone or device would have a different latency
you could work out the latency on the equipment you control ( bluetooth, h.264/5, network switch, access...
my similar size matrix of 24x92 (2208 px) I run on 2 falcon ports with 2x power injection at the 500px and 1500px marks @30% - no reason you shouldn't be able to run the whole thing without injection at 12v on 4 ports
welcome, I would suggest finding out what your local store stocks and run with that as far as connectors go - it makes that last minute I need 5 more of these plugs much easier to get, personally I run 12v, there is an argument for both, one is not better than the other jsut what better suits...
no of xlights, its just for pre made content - there are plenty of sound to ws2811 things on the market but you cannot control what they output or design any props