Those are pretty good tactics, but they are hard to accomplish by yourself.
It the Pyra has a competent ally, you are going to be forced off the grounded Pyra before it dies, and probably killed because pinning requires very specific positioning that leaves you quite vulnerable on most maps.
Try using Lochnagar rounds to get the final kill shot on the Carronade. Remember the Carronade is also REALLY good at taking down armor too! So the balloon being armored is actually an awesome thing for the Hellhound: by the time they hit the ground, the armor is already gone. But really, you should be focusing with your ally. Make sure they know that you will be taking their armor or balloon down. Make the match a 2v1. The Pyra is not over powered, it simply has a role as a heavy hitter. It's reasonably easy to dodge, unless you let it sneak up on you. My advice for that is simply "don't".
As for the heavy flak... well, all I'm going to say is it needs a little love. The LJs are very powerful, but difficult to aim. Remember, just because you're long range doesn't mean positioning isn't the most important aspect of the match! Sitting gives your crew more stable shots, but leaves you a massive target for long range weapons. Moving ruins the arcs/shots of your gunners but might avoid some ordinance. It's a balancing act.
But you're going to find out that this has been the case for so long that a lot of the meta for this game revolves around it.
That's not really true anymore. Pyra's are one of the easier ships to use by design. They are also one of the higher skill ceiling ships, meaning you see them throughout the game. The meta did revolve around gat-flak pyra's at one point, but only because they had a lot of armor. If you take a look at the 'state of the meta' gat-mortar is more common now. Pyra's just happen to have two light gun emplacements forward. But honestly, a junker implements that meta just as well, if not better. Additionally junkers have high armor, unprecedented mobility, and the ability to get three guns on a single target with little effort. Harder to fly, but more rewarding.
My point here is that instead of complaining that something is OP because you are having a hard time with it, take a careful look at how you are playing and how your ally is playing. When do you die to the pyra? Is it when your ally is near by? Why was he able to get his guns on you? Did you over extend? I'm not saying it's your fault, there is a massive continuum of skill in this game and sometimes you just get bad luck. Really though, I experienced the problems you are having when I started out. My advice for that is to practice and above all try new things. Hold back, let your ally go in and try to disable his balloon after he commits to not shooting at you. Stuff like that.