If it was an old spinny halyard then it was made of the wrong stuff- supposed to have a bit of ‘give’ in a spinny halyard are you not?
4mm dyneema is what I use, I used to have 5mm but didn’t find it necessary and it’s more bulky.
I have always found horn cleats to be more reliable than cam cleats. If it is appropriately sized and sufficient turns put on (locking turns if necessary) it will not slip.