Overall pretty nice guide for new - mid level. I can see quite a few things wrong but they are mostly situational things (like in the "who to heal" section, the answer is "whoever the enemy scout is shooting, and if they are shooting me I heal my scout for the movespeed). But as a general overview its pretty perfect at keeping ideas simple and approachable.
However the uber section deserves more love. The pre-post uber section is pretty important for medic as they are the only person who is in every uber as well as the person who controls the uber. I think there is a lot missing.
For highlander, ubers are not nearly as important as 6s, they are much less calculated and are much more about allowing a team to get into an advantageous position or for buying time on the objective. There are 4 guys who will get the medic on the enemy team if they truly want, and 4 more who can assist very aggressively as well. This is the natural thing when 18 people clash on a team based server. It is going to be less calculated and individual skill will have less of an impact.
In 6s however, ubers can be calculated and used as the strongest tool in the game. Glossing over them without explaining the intricacies is kinda just ignoring the basis of 6v6 competitive tf2.
The pre-uber section should be about the 5-10 seconds before using an uber. There are 3 situations for this:
1. Using uber aggressively with equal ubers.
2. Using uber aggressively with advantage.
3. Using uber from being forced.
1 is a team based decision, using uber to force a play to happen on the map, however numbers 2 and 3 are all about how the medic wants to play. If you are pushing with uber ad, do you want to get the point for free without using? or do you want to use aggressively to guarantee the point? The better a medic, the quicker you can get a point without using uber relative to using it. Checking sticky traps and hiding spots are things the team can do well, but ultimately dodging spam and surfing from bombers and positioning well from any threat is all on the medic player and that is what saves the time.
The other most important thing before an uber is to be healing the right target. The call is to go in and use uber so you shouldn't be healing your flank, use on them first and then use on the player who actually needs it. Being in the right place to actually use your uber is more important when under time pressure than topping up a buff for a roamer. Once again, the better the medic the more efficient healing of the entire team. By gaining more experience, these things will come more naturally.
During uber is all about your section of bad vs good ubers. However you never actually explained what a good uber is, what a bad uber is or what an uber flash is.
A flash is when the medic flashes the ubercharge for a split second on a secondary heal target, draining the uber and granting 2 people the invincibilty for a short time.
An uber trade is talked about as being "good" vs "bad" based on the length of the uber. At a low level that is all you need to know. Count the uber timing based on the number of flashes each medic uses and which medic used first. Whoever is ubered last has the "better" uber.
At a higher level, you start taking into account more things for an uber trade to be good or bad. You have to take into account the ammo counts, the health of the participants, the class of the participants, the ability to get good positioning post uber, the ability of teammates to come into the uber to assist and also the length of the uber to a much finer degree. A lot of high level players use all these things without even realising it. It is a form of gamesense summed up by "who has the easier time killing each other post uber" that determines who has the better uber.
For post uber fights, this was glossed over but is so important it even has a stat in the logs for it. During the tail end of every uber, every medic should be thinking about where they should be standing. In general, high ground good, avoid standing in choke points, avoid standing in front of your team.
As a team, there has to be a call during the uber if you are committed to the post uber fight or if you are leaving. This call has to come with sufficient time for the medic to either commit to a safe(ish) position in the fight or for the medic to backout of the fight and be completely safe. At a lower level, I would recommend the medic always give this call. At a higher level, I would recommend the most vocal person in the uber (ideally a combat class leading it) to give the call and if the call doesn't come at the right time for the medic, for them to acknowledge it and for them to then make the call. The reasoning for this is that at a lower level, the medic will be the most experienced person during ubers for their team but as you reach a higher level, everyone on the team will have sufficient experience to be able to lead an uber.
As you can see, the pre-uber, during uber and post-uber are all closely linked, with decisions and gameplay styles weaving between each other. Ultimately all stemming from how the medic and their team wants to play.
Ubers are hard to get right. Very hard even. Arguably the hardest thing to get right in the game. They are the best offensive, defensive, stalemate forcing and staltemate breaking tool in the game and is what forces competitive 6s to have the flow it does.
I could go on for a lot longer but I think this is enough for now.
tl;dr ubers are important and deserve more love in a guide about medics.