The video was good, and title not too much clickbaity (in a sense, is not those "You are playing the game wrong" or "You will not belive that..."), some good points too.
5e is one of the most popular edition, and the start for many people, including me. As a DMs, there is little guidance for new ones, but at low level errors don't matter. Forgetting a spell component, not reading well in a spell... happens.
But high level? There is so scarcity of teaching material, you can not simply "try, error, retry" in high level. An error can ruin a campaign. Rules wise the game supports high level (with one caveat: for me, every spell after 5th level should not be a level-up freebie, but should researched or granted with difficult). DMs need a book reference before entering to high level.
To me, a book of this should have:
1)General Advices on running the high level of the game
2)A run down of all spells of the game 6th level and up: What a party with that spell can do in an single phrase, what a NPC / BBEG can do with that spell, common counter tactics. Think of "The monsters know what they are doing", but with spells.
3)Same for magic items.
4)A way to prepare a "DMs Characters Sheet": a mirror of the players' ones but with information that helps a DM during the play, like what a caster could use, the common tactic of a melee player, and so on.
5)Lot of high level adventures seeds to give you inspiration
6)Example of high level encounters, combat and not.
That would encurage people to at least try high level.
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!
