To make an "all sides" Onslaught work, it either needs to have infinite flux dissipation it doesn't really matter and it can fire all its weapons all the time, or to have weapons so cheap to mount and fire that there's never a good reason not to fire them. The reason why broadside Onslaught might work better than the regular one is because of its constrained flux pool: it has only 600(+60+600+150) flux dissipation to work with, but if we made a "cheap" loadout with 3 hellbores, 5 heavy autocannons, 4 flak cannons, 6 vulcans and, obviously, 2 TPCs, then it needs 2781 flux dissipation not to gain any soft flux when firing all the guns, but just 1672 when firing only the forward firing weapons.
When working with limited flux, an ideal ship would have a priority list, where it would first fire most efficient guns and the least efficient guns last. Such a ship would not fire weapons past a certain point, because the most efficient guns used all the flux it has. A ship with a limited flux pool shouldn't fire more guns than its flux pool allows, because at some point it starts hurting more than helping. Even then, mounting any weapons uses ordnance points and if it won't (or shouldn't) be fired, those OPs are wasted. This means that you don't want any more guns than what you're going to use, and this number tends to be different to the number of mounts your cruiser or capital ship has at its disposal.
So, since you want to use the most efficient weapons, energy damage type is basically the worst type to use. Kinetic damage can be used against shields to double its efficiency, thus firepower you can put out with your limited flux. The same applies to high-explosive. This means that energy weapons of efficiency worse than 0.5 (which TPC is, at 0.
are undesirable. Firing 2 TPCs continuously for 500 DPS against shields at 400 FPS gives you less is worse than firing 2 Heavy Autocannons for 800 DPS against shields for the same flux. To make TPCs desirable, their efficiency would have to be boosted enough that they are higher on the flux efficiency list than ballistic kinetics or ballistic high-explosives for anti-shield or anti-armour work respectively. Or give Onslaught enough flux that it just can be wasteful with its weapons. This is exacerbated by Onslaught's low mobility, so it has to be assumed that what weapons it fires, it's going to fire indefinitely, without breaks.
tl;dr: Thermal Pulse Cannons can't compete with ballistics for efficiency and efficiency matters a lot to Onslaught.
As an aside, I hope Heavy Armour's manoeuvrability penalty goes away. It doesn't make it a hard choice, as ships that benefit from it the most, are hurt from the penalty the most as well, hurt too much.
