I was about to say I've never seen a sloe, but wiki says it's naturalised here so I probably have without realising!
I will check around in sloe season.
As the alcohol is the the preservative and I'm never in a hurry (sugar speeds these processes right up), I'd reduce the sugar to suit my tastes.
Also, no need to prick the fruit if your freezing them-
freezing expands the
water in cells, bursting the cell walls and doing the work for you