Thank you, very helpful.
I did some more digging and came across this tool which will tell you whether your intended runway gives you enough space to do a FLX takeoff, or whether you must go TO/GA, or whether it´s an invalid takeoff (it will tell you in red if the runway is invalid because it is too short). I think that solves the issue for me as I at least know what the correct settings would be, or whether I cannot takeoff from a particular runway.
I also came across another thread which links another tool below, which I’m yet to test: