Against the AI, attack is the best defense. Attack is actually much easier than defense.

For all my recent campaigns I only use peasants for garrison purpose, and haven't encountered any problem at all throughout every campaign. Peasants are cheap for upkeep for their size. You might already know that, only quantity of men, not quality, affects garrison effectiveness.

If there are captain-led rebels I just bribe them. If that's led by a brigand family member then I will wait a few turns to raise a small force, led by any family member nearby to crush them.

For frontline provinces, since you are constantly expanding, you really don't need to have an armored garrison. Your attacking army in that area will get rid off any annoying counterstrikes that dares marching towards your cities.