It's really too bad Yiski hasn't been able to show off his lumberjack skills yet. Soon.....
Letus as well is very adept at lumbering murder across the map.
On to the points. I will agree that the arming times for flak and lumberjack feel, reversed? I'd go so far to say that flak needs a slight reduction like the lumberjack got in the arming time to make it more viable. With its current damage against armor, I'm not very scared of flak at all unless its doubled up on a galleon with people who can aim it.
Beyond that though. As Smollet said, it takes 3-4 shots to pop the balloon with lumber at full health. The lumberjack's horizontal arc is a joke, so always approach them at an angle, and force the ship firing at you to turn. That in itself will throw the shots off a bit as well. Rely less on the arming time and more on the bad arc. Remember that being above it does you no good, as the upward arc on a lumberjack is insane, and you wont ever be too high for it.
Also, you forget all the bad qualities you get when using incendiary, like decreased damage and smaller clips.
If you really want to fight one at long range, well the best bet is having one of your own and shooting better. Otherwise, keep it disabled and make sure your pilot isn't charging head first. That just makes the shots so much easier.
The bigger problem here is, and don't take this wrong, but many people are hitting the point where running a gat/flak pyra and flying straight into enemies isn't going to work, and strategy starts getting required, not just something you can forget about and ram into everything for wins. And I'm not saying that to anyone personally. Calling something OP in this game is a big claim, and youll need a lot of support to really get Muse agreeing.
I dare to say the Flak needs a bit of a buff more than lumberjack needs nerfing. Even out the arming times to start. I need to use them more to see if it needs anything else. I will say that a heavy Flak shot on bare hull, resulting in a ship's demise, is a glorious thing to see.