Great post !
I'm always torn on the zone level thing, and it's always the place where having a leveling system feels like it eventually runs against a sandbox. I love playing sandbox, I have huge area like this full of mystery, wonders and blanks, and I run it as a huge hexcrawl. Obviously, that's too many hexes, so I tend to just have a very vague idea of what's there and when I know the players will go somewhere or are moving in a particular direction I prep a little and fill-in some blanks/hexes.
But it's very hard to decide ahead of time where things will be higher or lower level, especially since I don't have any particular limits for this campaign and it could very well go from level 1 to 20 which is way too large of a range for hexes. Either I include all the ranges and some zones will basically never be interacted with (good luck doing anything related to a zone 15-20 as a level 2), or the hexcrawl portion is a more limited lower range (say 1-10) and all the higher levels are specific locations, dungeons, villains, etc.
So far, I don't really decide the level of any given area as long as I'm not sure the players are even going there or near it. Obviously there's a general idea of what areas are more or less dangerous, but since i'm always making interesting encounters that stay fun for their level, it does feel like I'm going to have to scale things along with them.
How would you handle this? Do some areas basically become "useless" at some point? Let's say my players get to level 8, and I have an area near where they started that logically should be around the 1-5 range, then are the plots, encounters and things in it stuck there and incredibly easy? I guess some interesting and more challenging plots and locations can always be inserted anywhere, and the stuff that was planned to be low-level becomes more of a background that the players ignore or quickly stomp through.
These kinds of problems make me want to give a limit of 10 to this campaign, at least in the hexcrawl form. Beyond level 10 it feels like it's going to be very difficult to keep the crawling/travelling part challenging and interesting while also making sense. Can't have bloodstorms, hordes of zombies and super villains everywhere when travelling or the inhabitants would have all died a while back. So if they reach 10 the campaign would transform into more a high-stakes thing and the travelling part would be mostly skipped, I think that could work?