I am a fourth dan in Shorin-ryu Shorinkan and a fourth dan in Goju-ryu under Hokama Sensei.
Sensei Hokama says, "Goju-ryu is a bottle of water. Sanchin is the bottle, Tensho is the cap, and the water represents all Goju-ryu methods, all mixed." I like this analogy.
My Sanchin dachi and Naihanchi dachi are essentially the same ,both shoulder-width, with heels slightly out. In both of them, I am sitting heavy and compressed. I am absolutely comfortable staying or stepping. My weight is right in the middle. If I am pulled, I am not going to use my toes to stop it; I try to go lower.
The length of my Sanchin stance is toe to heel plus 2-3 inches.my back hip is pushed forward and engaged .
in my opinion, Sanchin is a fundamental kata as it is all about structure. Tensho and Naihanchi are not fundamental kata unless you do them and treat them as such; otherwise, both are fighting kata.
For me, Sanchin is about muscles and bones, and Tensho is about tendons. Naihanchi is my self defense kata along side Seisan.