Depends on the stage of the game and who I'm playing as. Last few weeks i've doing a portugal run through, and for ottoblob i've started hitting early-mid reformation age. i just take the war goal, capital, and then smash and grab forts while avoiding his troops, peace out around 50%. Between -20% warscore against other religions due to age of reformations, -5-15% due to Malta, and -20% due to diplo ideas, you can get close to your overextension limit + feed any spare to your byz vassal at 50-60%. My first war was me as portugal + burgundy PU and a naples vassal versus ottomans +tunis+irrelevent friends. First attacked some forts in egypt /tunis that caused him to retreat down there to defend them, when I saw he was resieging forts in egypt/tunis, I grabbed edirne and constantinople, then carpet sieged all of his european holdings. After he moved back to Asia Minor, I then smashed tunis for a quick seperate white peace, then peaced out byzantines for a province for a byz vassal, this was before reformation. Used naples claim to start the war. Second war for byz cores, using same tactics. At this point I could be more aggressive, but you still don't want to commit to fighting him without overwhelming advantages and the reward being a stackwipe.
Getting Corfu/Lesbos/Cephalonia/Egriboz/Sakiz are all extremely good for Portugal, even though Byz has no cores on them so you'll have to pay for them. If you put a fort on them, it can attract a otto-stack, which you then cut off using your ships.
Byzantines have to pay for the upkeep anyway, and like most vassals they suck at actually helping out except as a exhaustion sponge.
Also: by stacking warscore costs you can force vassalize an intact tunis in the age of reformation which is very useful, ottomans will go there and wander around, die in the deserts. Eventually they'll waddle over to Fez which you obviously should keep a fort + defensive edict on. Force vassalizing Mamlukes is also good to retake all their cores.
I don't even worry about his massive swarms of galleys (although it really is time to put the AI on the sailor limit: hell weren't sailors invented in game to simulate Ottomans getting knee capped at Lepanto by having their sailors massacred?).... just make sure you have a good admiral on your deathstack and your heavies +your insane naval tradition should have no problem wiping them out. The only time I really commit to open combat is if i catch him trying to siege Constantinople and I cut off his retreat; even then I use a merc stack to lead and don't commit marines to it. As a little guy, one bad fight with the ottomans can take you out for 5 years.
In general, its nice to have vassals you don't particularly like adjacent to the ottomans so he can wander around them and siege them down. Your vassal eats the war exhaustion, you rack up warscore, and you only lose dudes to attrition.
Definitely grab malta though, very nice and easy to grab monument..
ON FRANCE:
france is a monster but you can get a lot of strong allies who want a piece of it. Britain, Austria, whoever came out on top in Iberia, at least 2 of the three will join a war. In the first war take out frances strongest ally first, and your first peace deal is getting a few vassals like gascon, brittany, provence. Next war reclaim cores and you have crippled france.