I agree with those that mentioned Bad Guys. It's a really good and exciting action/thriller with killer action scenes, I really didn't expect to like it but the show proved me wrong and I was hooked. There's a second instalment but definetly start with the first, I myself haven't seen the second YET.
I also suggest BLACK. I'd say its underrated but I enjoyed it and it was definetly full of suspense and plot twists, I found my self binge watching the entire series.
I know you're looking for series but I thought to suggest some movies as well, you never know you might want a change of pace.
Memoir of a Murder and Forgotten. These are really good suspenseful thrillers that play with your mind and keeps you on the edge of your seat waiting to see how things will unfold.
Man from nowhere and Sunflower. These two are straight up action movies with the first being more so than the latter, but they really are so good with a GREAT storyline!
I would highly recommend you When a snail falls in love. It has all the things you mentioned => great action scenes, great thriller topics, and a cute romance too. It reminds me a lot of Healer just much better. C-drama though But truly this is highly recommended, I truly think you would enjoy this .