Thanks for the feedback! I'm still gathering player impressions and testing things on my end. The way I usually play him is by trying to maximize the number of digs I can get in before picking up a restore cell, so I don't waste the extra heals without growing his stamina stacks. By the end of some of my Pebble runs his max stamina can reach 170+.
I'll keep an eye on how players are feeling overall. If it still seems too tough for most people, I might lower it to 9 stacks for 1 stamina instead of 10.
Edit: I have lowered the stacks required for Pebble from 10 to 9 :)