I haven't really finished (yet) anything that's completed lately, but as for completed manga that has 300+ chapters, the one that came to my mind is Nanatsu no Taizai. I know you probably heard of this, so I'm going to share with you a recently completed manhwa I'm currently reading. I'm quite hooked up with it right now, but my schoolwork somehow did a good job preventing me from binge-reading it completely yesterday. Btw, it's the Legend of the Northern Blade, and I think it has 200+ chapters (?).The fight scenes are really cool and satisfying, and the MC is really something else! I'm literally procrastinating some of the incoming deadlines now because I'm really anticipating what will happen next after I read a chapter of it.