Both have a rich ML who is a cop.
however, in the good detective he was already a cop but got inheritance from his rich uncle.
In flex X cop he is an illegitimate son of a chaebol enjoying the rich life, but he causes trouble and is forced to work as a detective.
Both have a dysfuntional chaebol family that is an important part of the plot.
Both have their own traumas that affected them deeply and caused insomnia.

The kdrama and the Japanese movie series are so similar and give the same vibes.

both are about students locked in an isolated area they can't leave with one or more of them being "mafia" or "werewolf" that need to kill a person at night. "Citizens" or "villagers" have to vote out the mafia to win. They also both have a roles similar to police and doctor.

both are school dramas about a teacher who is a bit eccentric and airheaded. They both also seem to like nature.

Both are about time travel back to the warring period of japan.
There are several more similarities but i would consider them spoilers.

the Japanese drama is an adaptation from the Korean one
I personally liked both so you should try the other if you liked one of them