I think most people, including the people who wrote it, believe the DMG needs improvement. However, sometimes the best and only teacher is experience. If you're not very well versed in the game you should be starting play at level 1. By the time you get to high levels, you should have enough experience to play high level. You will all make mistakes of course, but that's nothing new and nothing that can be 100% avoided. Depending on budget, most of the things you list are best handled by a DndBeyond subscription. There is no way a book is going to keep up with new spells being added. If someone casts a spell I'm not familiar with I may take a moment to read it to ensure that I understand it. But counter it? Nah. I don't do that, what happens happens. Instead I build in flexibility, which is a separate topic.
As far as specific advice this is a forum where if you have a question you can ask for advice. You might even get some answers that will help!
I understand your point of view, but experience come from playing the game. I've mastered a couple of campaign, both from level 1. I've learned a lot, if I could re-run some sessions I would have mastered better for sure. But, after some time, with your PCs at eight, nine... one start losing control, if not trained. My players sometimes are confused by all the options, for me is five times the burden. I used the example of the "Monster know" books because, when I prep for a sesion and I know which moster will be there, is a good help to develop a strategy, and the encounters are fun (my players had to praise me). Yes, statblock and description are there, but that book help understand a little better thanks to someone who don't want to explain a rule, but how to use a rule. Spells are explained in the PHB and other books, effect and all. But why not have a book that can tell you that Simulacrum (for example) can be used for this, this and this. Can be really useful.
Having a more "trained" DM will help him to be more confident, and to run better high level games, and then the DM will gain real life experience, in a cycle of improvemente. But that cycle have to start, and not using an already going campaign as the training wheel.
Naturally I'm speaking as a newbie DM who started the hobby well in my 30s. Starting at 10s, with a lot of time, everyday playing and so on is something different. I recognize it.
I don't mean to pick on you, personally, but I've never understood the frustration some people seem to have with video titles. Judging a video by its title is, to me, like judging a book by its cover, or the tagline, etc. If there's an action story that bills itself as "the greatest tale ever told!" I don't hold that against it, because it's understood (at least to me) that it's not supposed to be taken literally, and so isn't trying to "bait" you into anything. And yet there are some people who treat "clickbait" titles like they were just sold a handful of magic beans (which didn't grow into a giant beanstalk).
/rant
Content creator wants clicks to improve their statistics/earnings. I just love when a video (but also articles) has a normal title that give you an information but also sparks the curiosity to see it.
For example, a lot of D&D content creators use so vague terms that they seem to insult my intelligence. "With THIS I started enjoying D&D again", with a thumbnail of a censored book cover. A click is not something I pay, and technically I should at least see two youtube advert before even knowing the subject of something. Why "pay" before even know what's inside. Would you buy a book wrapped in opaque shrinkwrap because "this is good"? Other creation contents would go to the tune of "Why I like Old School Essentials". I'm not interested in OSR or I already know OSE? That would be a pass. But if I'm slighty interested in it, and you are being honest with me, I will like your video a lot more. Naturally, that are my two cents too
