I have test mission for this sort of stuff - the variant was hardcoded.
I also had two railguns and two thumpers (Was using a cellphone keyboard, so didn't mention them). Basically, what I did, was I let the other onslaught unload onto my shields and once its flux was too high, it had to drop them once I started firing. Though they did sucky damage individually with low accuracy and range, all those mortars eventully burned through the enemy hull at practically point-blank. Plus I also attacked by raising my shields on top of the enemy ship, and since my ship was very economical with flux (both efficient shields and low flux weapons), and had absurdly thick armor and hull, I simply tanked the damage.
Awesome.
Kind of reminds me of a few times when I've started campaign mode with an underpowered frigate and ran into some overpowering enemies. I've often had to employ desperate strategies like wildly avoiding combat and letting them waste ammo, then supplementing my weak weaponry by ramming them with my shields up to help damage their armor and hull.
What you've done rather makes me want to see ship variants that are armored up and built for just slamming into other ships to cause damage, relying on superior defense.
In another campaign game I was playing, I found it interesting to load up a paragon with max flux capacity and dissipation, all the hullmods that enhance shields, flux, etc. and speed/maneuverability, and using it as a pure meat shield to absorb shots while my smaller ships dealt with the distracted enemies.
It worked suprisingly well, especially with a crew good enough to push the paragon to elite status, upping its flux/damage shield absorption ratio to like 0.3:1. I didn't have enough OP left to put any weapons on it at all, but it lasted forever before it had to vent flux.