Ive had the exact smae results as you Mike.
4 or 5 failed subs in the last 3-4 years.
No instructions for pea gravel, the last I looked anyway.
No extra grease.
Snap doesnt tell you about doing anything extra, its supposed to be a duried burial sub anyway, lol.

Every one I put in I clear silicone in and around both sides where the snorkel meets the tub and also at the 'waterproof' connector where the wire comes out.
I havent done all around the tub but it looks like that part is sealed well, the parts I mentioned is where I think the sh1t happens.

I actually have 1 right now I think is bad and was probably calling snap this week.

I am looking to move away from this system and looking at NEAR.