I think your tricep selection is poor.
Dumbells are not ideal for triceps. Cables are far superior and can be used in the exact same way.
something like (after a warm up of course, I prefer pushdowns for a warmup)
Smith cgp or HS dips
PJR pullover with ez bar
cable kickbacks or rope pressdown
if you like a 4th thing Id add a different cable attachment. 3 should be plenty though.
I see no way that dumbell skull crushers can provide the type of stablility needed for good progression.