well unfortunately it is the AI at the heart of most of your problem. If you want you can make your shells do more damage for when they do hit. and what you are experiencing with the DC is realistic as most any crew can hit a target at 1000m and if they cant well they need some remedial training

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the CFG files are easy to edit as you know but contain little to no information/ data on the environment and shell damage etc. If you want to edit those files you need Skwas silent3ditor.
I also woud like to point out (unlike call of duty) that the bullets/shells in this game are slightly affected by environment and do not immediately hit what they are aimed. so for aircraft you have to "lead" the target.