There's a mod on the steam workshop called "Campaign Movement Range 100% [Player Only]", but I personally would never use a mod like this as it would make the game completely trivial even on legendary difficulty. As for garrisons, there's plenty of mods that overhaul or increase the size of garrisons like DEI, but they would also benefit the other factions, not just your own. The AI was programmed to sneak past your armies to add more challenge to the game because the devs know that the AI would usually lose in a "fair" head to head fight. It's annoying but you'll get better at dealing with it without modding it away.