Oh, a fellow Onslaught enjoyer, nice to meet you

However, with regards to your build, I have some remarks.
First of all, it is not necessary to put a weapon on every single slot.
Second problem stems from the first one - you have invested heavily in railguns that have a shorter range than even the cheapest large ballistics, so they contribute less.
Thirdly, I believe that onslaught shound not be used as a broadside ship - simply because it will not be abe to also fire its build in cannons at the same time. And if you do go broadside, its for the best to use only one side of the onslaught for main cannons, as the opposite one doesnt have the arc to reach.
As for the hullmods - a lot of them I have also used.
Here is the build of my 4 onslaughts:
60 60 vents caps
S Hardened shields
S Heavy armour
S Reinf bulkhead
Integrated targ unit
Aux thrusters
Armored weapons mount
Stabilized shields
Shield conv - Omni
Expanded magazines
Resistant conduits
Weapons:
Gauss cannon
2 heavy maulers (central mounts)
2 devastators
3 double flaks (front middle, and 180 mounts on the sides in the back)
4 flaks (2 front on sides, 2 in the engines in the back)
Skills:
helmsmanship
target analysis
impact mitigation
ranged specialisation - elite
shield modulation - elite
gunnery implants
As you can see, there are no small weapons installed, as well as no rockets. The reasons for not putting any missiles are 3:
First, I simply cant afford to cut 40-70 OP from other weapons/hullmods. Secondly, I have designed the built with the intention of prolonged battles at artillery range, keeping the enemy as far as possible - the rockets are just too short (like annihilators or reapers) and they will run out of shots quickly. And thirdly - if youre gonna go missile, youre best bet is to go as heavy on rockets as possible - simply because rockets are shootable. (if you shoot 1 missile, it will be shot down, if 2 - youll break shields, if 3 - only then youll deal damage - you get the idea).
Gauss cannon turns into a godly weapon with such a build - but its important to have the elite ranged spec, for that extra projectile speed that makes the shot inevitable to all but fastest targets. And the nerfed impact mitigation also boosted gauss cannon, as before the shot did practically zero damage if the enemy ship had the skill, and now the gauss cannon, even being a kinetic, is a all rounder weapon - each shot is so powerful that it still shreds all but thickest armour of capitals to pieces. Even the -10 weapon range in remanant battles is no issue for gauss cannon, the range is simply that big.
It is important to mention that these 4 onslaughts work in tandem with 4 fully vented monitors, and 2 moras (with lux fighters for light support/distraction), and a piloted distraction
carnifex afflictor. Moras and afflictor are not that important, but monitors are essentially a part of the onslaught - kinda like the spotter and the artillery cannon are parts of a single weapon system irl