I would also add to what doughy said that medics should make decisions themselves based on what is going on in the uber, if theirs is shorter and things don't look good to them, call halfway through the uber that you are leaving. Relying on a combo scout or demo to call post ubers is a pretty bad idea, even in invite it doesn't happen that often and it will screw you over on a new team.
As a medic you can see who your scout and demo are shooting, see the enemy team much better as you aren't focusing on killing things, and see the health of your teammates (and enemies thanks to solemn vow), the only information missing is what your combo scout intends to do. By all means if the scout or demo wants to collapse or chase people after the uber is over they should call that, but most of the time it is better for the medic to say what he intends to do (e.g., commit top left, tell scout to play point, etc.)
I think in general as a medic it is important to take advantage of the fact that you aren't a DM class and don't have to focus on aiming, and use that to dictate how ubers are going as a stream of consciousness, giving as much information to your team as you can since it is miles easier than the combo scout who is trying to kill people as fast as possible. Main calling pushes and commits during dry fights and stuff like that definitely goes to the maincaller, but most things uber related can be called by a medic and if you practice it and get a good feel for it, it will help your team a lot.
Also I feel like the best way to do this would be to have this initial guide, turn it into little video chunks (with topics for each one so you can skip what you already know), and then get experienced medics to talk about more advanced topics like intricacies of ubers. Nursey had some really good map guides for medic but then deleted them all so there is definitely an opening for content like that, I think if you are keen and willing to do some editing and organizing you could definitely get people to help you out with things.
Marxist's precepts of medic and '10 ubers' videos were great and something more updated to scout speed meta would be super helpful. If you found examples of stuff like good and bad pushes, good and bad ubers, baiting too hard vs not baiting enough, etc. and recorded them I would be happy to talk about them and say what was done well/poorly (plenty of other meds like skeez would probably be down as well if you reached out). TF2 is so fast paced and there are so many little details that I don't think can really ever be conveyed by a bunch of paragraphs, the best way to help people is definitely to do videos with examples for more advanced things.