Model is Senheiser Momentum2 HD1 Bluetooth (over ear) PF edition.
I have wrapped electrical tape over it but cables on both sides have started to disintegrate/decompose.
I know next to nothing about audio cables. I care more about functional quality then original looks so i would be willing to swap the entire cable for a more permanent one, but i wouldn't want them to look ugly either and need to find a good way to fit them to the earshell.
In addition to the advice from @beeng@discuss.tchncs.de, I would also say that stranded core, silicone wire will be the most flexible and will be more than durable enough. You shouldn't need too thick of a wire gauge. AWG 20 should be more than adequate.