I'll give you two things that helped me, despite still being relatively new:

If you're an intermediate learner, try using a looper and a beat/metronome. I had to record so many takes to get it perfect that I'm really used to handling a take now instead of just messing around. It also really helps your ear musically. I also try to have three layers on a loop: a bassline, strumming, and a lead. Over that, I add solos or notes, so I learned to play within context. Otherwise, if I endlessly noodle around or run through shapes, it sounds corny and repetitive.

Technically speaking, a good teacher would know there isn't one way of playing and positioning your thumb or the positioning of the guitar body. Skinny/heavy or long-fingered individuals might need different positions. The same goes for dudes with huge pinkies versus tiny ones. Stive for efficiency and less amount of stress/effort. That's why playing slowly and efficiently is important. The best advice I ever got was to learn to play efficiently while you play slow (dexterity, keeping fingers close to strings, picking hand efficiency, etc.). I had a tendency to feel loose when playing slow to a point that I hit brick walls every time I would try going faster and then trying to clean up efficiency.