Being that the Conquest is in fact my favorite ship, I'd be happy to give you some advice.
You're pretty observant, so you've noted a lot of the Conquest's weaknesses, namely that it's flux capacity really gargles nuts. If you're playing in dev mode, you can mess around with its loadout, and I personally dropped the Mjolnir cannons for Hephaestus guns, because really, those things flux up way too hard.
But if you're not, I still do have some advice. You're right that the shield turns slow, but its' coverage isn't bad and the Conquest actually has some fantastic mobility for a ship of it's class. Always move with the pack, and don't be afraid to reverse. If you find yourself in need of redirecting your shields, a trick is to no try and turn the shields but instead drop them and restart them in the area of your choosing. You're trading coverage for location here, but coverage doesn't mean a damn if it's in the wrong place. Vent WHENEVER possible, and often. If there's a Lasher or a wing of Talons around, don't even give a ***, just vent.
The spinal torpedoes are probably your most important asset. Two volleys of those will jack up anything, Onslaught or not. So your approach should be a question mark; come in with your spinal mounts facing the enemy, and launch your torpedoes whenever you're confident in having them hit, then break to a side-- preferably the side with your forces on it and swing around, letting your powerful broadside hammer the sides and then the back of whatever you're hitting.
Part of your frustration is definitely that the Conquest isn't really set up right for the Last Hurrah, where the biggest problem, honestly, is the fact that you get zoned out by fighters. It's got those Mjolnirs, and it hasn't got enough in point defense. But take my word for it-- one of these days, you'll burn in on an Onslaught, blow his shields away with your torpedoes, come around hammering him silly with your broadside, as your wolfish grin widens-- the firing arcs of your torpedoes lining up with that fat bastard's engines..... and you will enjoy it heartily.