I don't think Onslaught needs to be buffed. It's pretty good, but it has its weaknesses. The only buff I really want is the return of burn drive cancelling and to make AI capable of using it. And if you need an even bigger low-tech ship, Paragon plays almost the same, just with good shields.
Though I'm not a fan of TPCs. They're nice for burst, range and being free, but if I could, I would choose the old reliable kinetics+high explosives combo. Gauss Cannon is the only kinetic ballistic weapon less efficient against shields than TPC and against armour it's probably worse than all medium and large high-explosive ballistic weapons. It's nice for opening combat, before your other weapons come into range, or for finishing fights quickly so that overfluxing yourself doesn't matter of the enemy is dead, but most of the time it's best to avoid shooting it. Onslaught doesn't have the flux for that.
Yeah you're right about TPCs, nothing else the Onslaught can mount can compare to them. I disagree on low tech being cost efficient tho, their ships eventually start costing more than other techs in the long run, especially if you tend to take armor or hull damage. The only thing that makes them seem cheaper is the lower credit cost upfront.
Eventually? Conquest and Odyssey are cheaper at 300k and 330k credits respectively.
Fuel costs and crew salaries are the real killer.The changes would make Onslaught a terrifying sniper platform - with 3 converging Gauss Cannons and flux buffs what would be the point of Conquest? So getting close to enemy with Burn Drive wouldn't even be that much of priority.
Running away. Slow ships can be overwhelmed, even the Paragon. On the other hand, Conquest can easily run away and keep shooting you while at it.
Onslaught is stronger than Conquest, but Conquest is faster than it overall. It's Onslaught's job to fight Conquest directly and it's Conquest's job to avoid a straight fight.
(I think it's actually pretty close if we uses zero flux boost because that stacks with maneuvering jets while it doesn't stack with burn drive)
You need Helmsmanship 3 to get zero flux boost with burn drive, which is one of the rare instances where the skill gives the player an actual benefit, instead of being just a patch for AI.