I think if everything breaks your way, you could, in theory, defend against a superior force using the mountainous terrain in the balkans. That being said, if your goals are to 1) defend against Germany and/or USSR and 2) remove kebab, you're probably going to fail. I find it highly unlikely that you would be able to get enough tech to defend your northwestern border using mountains, then also push eastward through largely mountainous terrain. Could you conquer the Bosphorus? I don't see why not. I think you would end up expending too much effort to push further east though, and causing your western flank to crumble as you push resources to rearm and replace divisions even with a relatively weakened Turkish army. If you completely shatter the Turkish army at the Bosphorus, you may be able to expand to the Caucasus, but I still don't think you have enough manpower, production, or resources to commit to defending 2 fronts en masse, which would be necessary. I would also guess that you would have to fully embrace Facism to be able to conquer Turkey early enough for it to be of any use, which leads me to believe you'd be facing Turkish revolts after even minor setbacks, as you're already going to have to give up some land + national unity to retreat into your defensive line, which would be detrimental. You're also working with the southeastern Balkans and Asia Minor, which are not nearly developed as most of western Europe. You also don't have the bonus of someone like India of having a massive population.
Tldr You could defend against Germany/USSR (difficult) or conquer Turkey (fairly easy), but conquering Turkey to then defend against USSR and Germany on 2 fronts? Doubtful.
