I research fighters ahead of time and build a lot them

Can't advise on how to beat them in the air unfortunately, but their production of planes can't be judged by the number of factories they use to produce planes. Production in Black Ice is handled quite differently for the AI. I can't recall the specifics but it's mainly stuff in the background that handles their production.

If it's a mod that cares about realism, people *don't* do a successful Sealion.

And if they do, the developers of the mod have work to do.

Build radars on the french coast, they will help a LOT. Also i notice AA kills, do the bombing runs too attacking their AA, airfields and radars.

Ignore GB until you have capitulated the USSR.
You need every fighter to slay that giant ASAP
I usually capitulate the USSR mid 43 and then you have the free IC thats necessary to end GB.
Wasting planes before barb will cost you dearly

Aircraft is horribly imbalanced as Germany for sure. Even if you invest absurdly heavily in aircraft production from the very start, it cripples many other areas of your construction/production in the process as you also need engine/aircraft plants built as well, even with what you get from focuses. If you even remotely try to build other things like road, rail, refineries etc., you will instantly fall short. Realistically German industry was hard pressed to keep up with equipment/resource demand in most areas, but this is absolutely ridiculous, and - historically - Germany was never less than on par at the least with the Western allies vis a vis the air force at the onset of their Western offensive. The English and French between the two of them had over 10 thousand aircraft come May '40 - 6000 of them land based fighters - in my most recent play-through. Terrible balance, and completely unrealistic for them to have that much aircraft/capacity for production in '40 - even with colonial contributions, if that is a thing as there are apparently hidden production inputs for allies, also imbalanced. Incredible fail in such an epic mod, and work still needs to be done to address this imo.