Payback

- one main had to get involved with the mafia and became a loan shark and his love interest is an equally morally grey enemy and they have prior history
- there is some revenge and rival team up undercover work
- it's dark, contains action scenes, betrayal and violence
- dips into the entertainment business at times (Payback a lot more)
- enemies to lovers

both have themes like
- mafia
- money problems
- loan sharks
- death of family member
- main(s) who has fighting skills
- entertainment business (acting for Payback, boxing fights for Knock Out)
- an ex who caused problems and is now a rival
- enemies to secret lovers
- action and fight scenes

both have in common:

- main character named Sun is involved with the mafia
- people he loves get killed
- he swears to take revenge on those who are responsible
- infiltrates the rivaling side to do that
- but gets romantically / sexually involved with someone in there
- generally petty rough, dark, violent
- have action fight scenes
- secret, forbidden relationship
- main has an ex and the parting was very painful
- main couple do some fight trainings together
- kind of enemies to lovers
- explicit (partly rough) sex scenes

differences:

- PayBack takes also place in the film industry with shady, illegal elements
- main couple starts with power imbalance and sexual favors (so far based on episode 1 and trailer)
- Bangkok Boy stays in the mafia realms
- main couple is very hurt/comfort and pretty equal

Both are similar because it story revolves around film industry and sponsorship issues.
Both male leads wants revenge
Both male leads get helps from higher position male leads..
Their chemistry soo good ?