You know, I feel there's a much simpler solution:
Adjust the maximum area level you can access to be civlevel * 2, instead of civlevel + 10. Then, the equivalent at level 300 of raiding level 40 regions at level 30 would be raiding level 400 regions.
That is the worst sentence, but I like how bad it is so it's staying. A better format would be "at level 30 you raid level 40 regions, and so at level 300 you raid level 400 regions".
Anyway, that keeps difficulty relatively static. The solution for the display of numbers getting out of hand would be to just use the k, M, etc. suffixes for numbers, so you do 3M damage instead of 3,000,000. People might be confused a bit when they're doing 185Z damage, but they'll figure out through context that it's 1000 times more than 185E. Any rounding errors are also likely to be not particularly important.
Oh, and it occurs to me that the resources you find might have to have a nonlinear scale as well, so that you are getting approximately the same relative reward for +100 levels at level 300 as you got for +10 levels at level 30.
The problem with this is exactly what was fixed with the patch, I believe. At first it makes sense-- why not have a level 800 area twice as hard as a level 400 area, if a level 8 area is twice as hard as a level 4 one?
The problem is that as your levels get progressively higher, they mean less and less
. Going from civ level 3 to 4 is exciting because it's a big step. Going from 400 to 401 would mean nothing at all, and suddenly the XP bar becomes arbitrary compared to the level number itself! So eventually you start gaining "microlevels" and your perception of real levels becomes groups of 10, or 20, or 100... suddenly you don't say "I'm 50 percent to level 20!" and you start saying "I'm 25 levels away from level 400!"
I'm a huge fan of this new relative system. It makes it feel more like you're unlocking areas through your effort, like a more traditional Metroidvania game, rather than just boosting your numbers high enough to beat the other numbers!
One minor note that's almost on topic. The only thing I don't like is liberating resource spots that are waaay under my level. There's no challenge but it still needs to be done, and most of it is just walking across the surface. (Maybe there's a command to make NPCs do it for you at a certain level? I'm not sure.)