You are correct in not looking at the agency if the instructor is a good one. I actually never mentioned the agency in my post
Indeed my point is another one, but maybe I was unclear, so let me rephrase a bit.
Tech training is rather more intense than rec training; skills are essential, and often, if a diver learns "wrong" skills, it is hard to correct them. Therefore, it is better to be reasonably sure that your instructor is good. Even personality plays a key role for obvious reasons.
Now, about personality, you already like that instructor, so that's half of the way. What about the other half? In other words, is he a skilled and capable instructor?
If the only tech instructor you know is him, it will be tough to answer this question because you do not have any references. Therefore, I would contact other instructors and chat with them to make a comparison, especially about the course's content and the level of skills that other instructors require to pass the course. I would also contact some tech divers who did not pursue any training with him for the same reason (quick chat with these divers, and then compare their courses with the one of the instructor you know).
At that point, you have a reference, and you can get an idea of how good the instructor you know is.
That said, it is only a suggestion, as it is the one of pursuing cave diving training before tech training (for many people, it is a better path, and it's cheaper, but it depends on the person). So feel free to do as you want