Captain's discretion is exactly what it says.
Very tricky to decline a flight departing home base on a multi sector day claiming you will enter discretion as that will only be the case on the final sector. You can then say no and get a lay over at the hotel on minimum rest. I know of a Capt that packs an overnight kit for just such an occasion.
I have refused discretion departing DXB on single sector flights 3 times and have never heard anything from HQ either during or after the fact. A lot of pressure but if sticking to what you believe then crewing will back down.
Just have to pick your battles and avoid the ones you will never win.