From your own maps and paintings, those brushwoods are not very thick, they probably slow moves but not stop them....
It is possible to watch actual movies from WWI and even WWII, with guns horsedrawns . I also saw real ACW pieces in reenactement closely...
They move at an amazing speed ( in emergency ) and on very broken ground when necessary. They were only stopped by thick woods or hedges and by straight cuts, and the gun teams managed to clear paths in wooden bareers or push a fallen tree aside quickly.
The guns like french 75mm or german 77 or ACW "Napoleon " had also limbers and spiked wheels and the gross weight was about the same as average 8pdrs... So, I 'ld call it difficult /slowing ground but not impossible
Oster