1. Strength of a trainer doesn't matter and it doesn't mean you will get effected by their strength and grow faster lol.
The strength of the teacher does in fact matter
- The stronger the teacher, the more experience the teacher has, thus making them able to teach the student more. As for Hyogoro, he has experienced Adv CoA, that's why he could teach luffy, it still even needed outside interference (BM) to trigger Luffy's Adv CoA
- Hypothetically, the student can get stronger by fighting the teacher constantly so as to grow stronger, now i ask you, fighting Ivankov and fighting Mihawk/Rayleigh, which fight will make you grow stronger in the end? Because i think this is the case for Zoro, his permanent scar i think was due to his fight against Mihawk, because i dont think oda will make a mere baboon leaves a permanent scar on someone as important as zoro
2. Zoro will never be stronger than Luffy - pure headcanon. But Zoro isn't stronger nor as strong as Luffy, so no problem there.
There's literally no ground that can backup "Underling will never be stronger than the captain" statement lol, i agree luffy > zoro tho as of rn
3. Luffy has the fastest growth rate in the whole crew.
yes, so does zoro (at least in my eyes)
4. Most are training everyday, we only see Zoro train because there is nothing else to show from him. Every time we see the SH's on the ship, they do their jobs, Zoro doesn't have a job like the others, he doesn't do anything besides fighting. He is a combatant. So the only thing you can show him do is training or sleeping.
Headcanon