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.