I have the same problem: finding a k-dramas with a great romance, no-jerk hero and no love triangle can be a real challenge! Granted, I am a beginner in k-dramas as well, but from my limited experience I can safely recommend the following:
1) Healer (don't be discouraged by the thriller tag on MDL - I almost skipped this drama since it was not marked as my usual rom-com stuff)
The plot itself is not the easiest one to explain, so I'll just mention the romance's highlights:
- Superman-Clark Kent-Lois Lane kind of romance, aka it sucks being jealous at your secret identity XD
- all misunderstandings between the main couple are resolved rather quickly, they actually communicate (how rare is that in the sea of k-dramas?)
- fantastic chemistry between the main couple, seriously, I have no words to express how cute and amazing these two are together, just go watch it for yourself, please :)
Negatives:
- a bit slow at the beginning, can be a bit confusing as well since there are flashbacks to the 80's and their significance is not initially clear; anyway, it got really addicting for me around episode 5
2) Queen In Hyun's Man
- time travel romance with a happy ending
- super cute main couple
- shorter episodes (45 min each)
- likable, smart hero who finds himself transported 300 years into the future and adapts to it pretty quickly
Negatives:
- there are some annoying side characters in heroine's time but they don't have a significant impact on the main relationship
3) The Master's Sun
- the heroine sees scary ghosts, has no life because of it but still does her best to help them and to find her place in the world
- again, great chemistry between the main couple, many hilarious moments
- the show has a solid beta-couple as well
Negatives:
- the heroine can come off as "pathetic" and "clingy" at the beginning but really has her reasons given her circumstances and otherwise, she is really cool and strong-willed
- the hero kinda belongs to the cold jerk with a tragic past category, but he gets sympathetic fairly quickly and he and the heroine balance each other well I would say
4) Splash Splash Love
- drama special, only like 2 hours long in total
- very cute time travel romance: predictable, but nicely executed
- great if you're just starting with historicals
Hope this helps!