But if your only goal is to hold out as long as possible, why put artillery on them and not units with more strength? 4x Garrisions divisions should have 12000 strength which is 50% more then your setup, and also need 100 less leadership.
I only use artillery when I know the front will always be filled with frontline brigades. Like in very concentrated attacks or defences.
Seconded. I tend to believe using Art on static defence is a waste. When I am using GAR/MIL as border guards, I field them exlusively. 1 or 2 Inf+3Art divisions will be stationed behind the line as fire brigade. Port defence usually 2GAR division is enough to hold position until reinforcement arrives. IC/Airfield places solo MP brigades.