FWIW, you could probably field 5 Pillagers for the same cost as those fleets, and at least under the old balance I know a Pillager build where 3 could be enough to deal with the high tech fleets and 3 or 4 could handle the low tech fleet.
Different fleets play to different strengths. The way that you've set those skirmishes up should play to the strengths of high tech. If high tech is losing in that short of skirmish scenario, it doesn't bode well for them strategically unless they maintain a significant advantage in the carrier space.
High tech needs speed and space to thrive. They want to spread the battle out into a number of isolated engagements, move in an unload doing some lasting damage while only taking flux damage, then disengage to reset. Sending them up against even numbers of Dominators and an Onslaught should be perfect for them.
If Low Tech is going the slow and ponderous route, then they need to deploy combined arms: they need support from missiles / artillery / gunships to sustain flux pressure. It's like advancing infantry behind a creeping artillery barrage. Their slow-arse Onslaughts need to push the high tech back to the edge of the map.
The other alternative is to take away the biggest advantage of high tech--mobility. The Pillager's burn drive allows them to adopt the Nelson doctrine and "engage the enemy more closely." You have a 22 DP battlecruiser that can delete 17 DP Auroras--or at least it could under the old balance--and go toe to toe with Paragons, as long as it survived the Paragon's initial alpha.
But your skirmishes, imo, should favor high tech unless macro balance has been seriously reworked.
My Pillager build was roughly:
- Mods: Frontal Shields; Advanced Turret Gyros;
- 3 central mediums: Heavy Photons
- 1 dorsal large: MarkIX
- 2 forward large: Excalibur cannon aka best weapon
- 2 wing large: heavy Vulcans
- 2 forward wing small: Photon cannons
- 1 rear medium: Vulcan
The rest is mostly PD and some interceptors to keep the torpedoes away.
Now this build may not be as effective without the captain skill for improved target leading, I'm not sure. But it's effectively a zero flux build with tolerable shield efficiency, burn drive, heavy armor, good range, and very good sustained DPS. It's more than twice as good as a Dominator and costs only 150% more... and frankly, it's enhanced mobility makes it better than an Onslaught as far as I'm concerned. The Pillager will jet around the map almost always shooting at something.
In skirmishes, as opposed to fleet engagements, the Onslaught will spend most of its time wishing there was something in range for it to shoot, and when it finally does get in range of something, it will use its built-ins at the edge of the shield box. That's the real problem I have with powerful weapons with long CDs. The AI is dumb and aims for the edge of shield boxes rather than center of mass. That means that 20-25% the time they fail to damage armor/hull, even if they are "on target" in these sorts of isolated duels. I did fly around with a pair of salvaged Onslaughts in my Luddic Path playthrough, but I only brought them in for major fleet engagements. Usually 2-3 Pillagers did all of the heavy lifting. Since I was playing Pather, I felt obliged to use Pather Eagles as meat shields... instead of just more Pillagers. Plus, I had a very limited supply of Heretical Persean weapons. I had to pay the iron price for them, after all.