Some stuff I would do to fix 5e

[Psyren]

But how would someone know this, and should the guidance be spread out like this. As an example, I don't own the Ravenloft Campaign book.

The advice is in Ravenloft because it was the first nonstandard campaign style they introduced to 5e (Dark Fantasy/Horror). The DMG explicitly advises you to use Ravenloft game products for this type of campaign on pg. 40. And that's how all nonstandard campaign styles, i.e. anything that deviates from Heroic Fantasy (the default for 5e - DMG 38) work; the DMG briefly describes them, but you're expected to use other game products actually set in that campaign style to flesh them out, relying on third party or homebrew if nothing official exists yet.

For Sneak Attack, I tend to agree with Witty Username in that it seems more a method of fighting over the types of weapons.

Some methods of fighting entail or invoke a type of weapon in order to function. Just like Great Weapon Master assumes heavy two-handers that use Strength and Sharpshooter assumes projectile weapons and darts, Sneak Attack assumes finesse and ranged.

I agree that level by level multiclassing is obnoxious. Part of the reason though is that class features taper off dramatically by end of tier 2. Classes will always be frontloaded, I think, to get characters up and running in the game. But features should stay impactful and interesting all throughout, instead of feeling like the devs are throwing us a bone or reaching to the bottom of the barrel to fill a level in.

I'd say that rather than removing level-by-level multiclassing, the better solution is to make Tier 3 and 4 class features more desirable. Multiclassing will still be the ideal choice for many builds, but it wouldn't be as automatic as it is now, and become a true choice. All the martials are getting upgrades along these lines, that make straight-classing much more attractive for them.