@boscoman it’s not the thickness, it’s the model, change to a top of the line O’Neill like Psycho Tech 3/2 or similar and you won’t need any layers underneath. For upper 50s and higher, this is all you need. Zero leaks for the first 3-4 years or more, stretchier than the Epic and warmer. You can go for a 4/3, but for upper 60s there is definitely no need.
Both my Psychofreak and psychotech are leaking now, but still comfortable at upper 60s. Need to buy new, have used them for way too many years now and don’t think there is any life in them anymore.
I could do 50 when they were new.