Hi,
first impression: cost is rather high to get a limited protection from some flanks.
I understand that giving all Inf and Art a +D1 is quite powerful, this explain the cost.
But if it is too high, then people will prefer to buy 2 Art units instead, for the 8 IPCs.
A cheaper and less powerful somehow can help selling the new unit.
Maybe 2 types of fortifications can simplify the border problems:
Sz fortifications (give a bonus vs any Amph. assault) and inland fortifications (give a bonus vs any invaders coming from any adjacent land territory).
To provide a defensive bonus to Inf and Art seems an accurate historical HR.
In essence, it makes them at the same level than a Tank unit at D3, but with no mobility.
The basic cost for this bonus is around 1 IPC/unit since Art cost 4 and Tank cost 6.
It can be halved since it cannot move and have no offensive value. 1 IPC/ 2 units, or (.5 IPC/unit).
Or it can be seen somehow as similar to Air Base and Naval Base because of the 8 IPCs cost.
Giving an inherent AA gun capacity, damage points and susceptible to TacBombing Raid and repair.
Can be captured, plus other bonus…
Just my two cents, for now.