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.
I was reminded of the "Fist of the North Star" which is a totally different show but triggered by the word north. Some golden age anime.
The panels look cool~
Ahh I see... I'm not really familiar with what you've mentioned, but it seems it's also about martial artists. This manhwa is also about different kinds of martial artists—from those who use bare hands and fists, who use swords and different kinds of blades, to ones who specialize in special techniques such as poison and mental abilities. It's really a cool manhwa, and it totally kept me hooked up.