Tried some cheap rovex 60lb a while back for shockleaders had a lot of problems with it breaking even before my mainline which was about 30lb. Generally I use 80 – 100lb blackmagic leader line but if I’m only fishing for smaller things and need the casting distance I use 60lb. Depends on the size of the guides on your rod as well cos a lot of surf rods have quite small tips and you get the knot knocking on the guides a lot, fix for that is usually just upgrading the last 2 or 3 guides on the rod to a size up.
I wouldn’t reccomend a double uni as a strong connection for your shockleader, there’s a few others that are quite a bit stronger. I think most people use the blood knot which is pretty much the same knot you use for making dropper loops on a normal trace and works well for most people although not the strongest knot. I use a Centauri or fig8 knot I think it’s called mainly because it’s one of the strongest knots and I’ve never had my line break at that knot through fighting a lot of big fish and if you double your mainline for the knot using a bimini twist it makes it pretty much the strongest connection from mainline to leader you can make just becomes harder and more time consuming with the bimini twist which is why I don’t bother most of the time.