You are conflating different things.
The EU accession project was abandoned long before Syria, when it became clear a lot of EU countries (Germany above all) were not going to allow it for fear of increased migration numbers. Since then, Erdogan dropped any pretense of liking the democratic process and the rule of law, which were supposed to be strengthened as precondition of joining the EU. He used the real issue of corruption in the secular army as an excuse to strengthen his own position.
The kurdish issue is a long-standing problem and Erdogan uses them for classic "pressure management ": whenever he's under threat, he whips out the kurdish enemy to solidify his support. He's done this pretty much continuously since taking power, making a bit of progress on peace efforts to keep up appearances then cracking down whenever he needs a popularity boost. Like black and red terrorist groups in '70s Italy, kurdish groups are likely infiltrated at all levels by authorities.
Erdogan's wobbling on Syria had little to do with alignment with Putin or the US, and more to do with him initially betting on ISIS then backtracking once it became clear US and Russia weren't going to accept it anymore. The Russian plane shot down and subsequent apology was the end of that particular game, nothing to do with long-term alignment. Turkey had already denied their US bases for a number of operations before then.
Erdogan is not a pawn or even a solid ally to this or that front, he just plays his own fascist game.