I wouldn’t call the Graf Zeppelin a “light” carrier (let alone CV or CVL, which terminology is used by the US Navy alone). At 33,550 tonnes, the Graf Zeppelin was of comparable size to other WW2 fleet carriers. It did carry fewer planes, though.
If it had been completed (and I don’t say that doing so would have been a good idea in the first place), I think it would have been best used in Norway alongside Tirpitz. Tirpitz was a headache for the British not because of what it did, but because of what it might do, as they constantly had to take its existence into account when operating in the North Atlantic. The planes on Graf Zeppelin could have provided a similar threat. And of course, those planes would have been very useful to protect the ships when the British came to bomb them.
About the Bay of Biscay - I don’t think a German carrier would have been any good there. If Germany lacked the capacity to protect its U-boats with land-based planes by 1943, anything else wouldn’t have worked either.
Apart from all that, more U-boats would definitely have been the best naval build for Germany, imho.