Think about the tactical limitations of planetary proximity and the XY plane VS open space, whilst allowing for forward motion and evasive moves and you have your answer. e.g easier to put both guns on target whilst running evasive at flank speed and thusly not die.

Hmm, I may be wrong, but why is the Donnager tilted 90 degrees? Is it supposed to be like that?

7 hours and 300K of pretty...
At around 300 000 Materials and just under 100 meters .
The main engines set to 45% until enemy contact to save fuel, this will push the engines to their limit for 8 seconds while the main powerplant comes on line.
Composite armour hull, heavy armoured EMP shielded ammo and AI.

X2 large long missiles
X4 medium long missiles.
20-ish dual simple cannon point defence
X2 250mm 1700MS-1, 70 round per minute railguns.
Made mainly for looks; Once they start firing the weapons sound like they are bringing hells fury to bear on the enemy, however they are somewhat underwhelming.
65MS-1 cruise speed.
Multiple heavily armoured sensors.
Single core AI.
Medium as regards campaign units.
At some point I might replace the experimental rapid railguns with alpha strike, long reload versions as per Randompros advice.