True, taking Avamar was an immense undertaking and holding it was a logistical nightmare. However later when Fontan's military was already weakened by the Civil War we still raided as far as Glinmar on two separate occasions and that was with the military command actively opposed to the ventures. With their support such a strategy might have borne more substantial fruit.
A realm in control of Ashforth and with a willingness to use all forms of warfare in combination could certainly force the passage and do serious damage. It would still require good planning, a large element of luck and a disciplined military.
