Very similar setting, hospice, past connection between leads but Chocolate is more "slow burning" on the romance side. Both involve some cooking. Leads in IYWUM are nicer, honestly.

The vibe is very similar and there are strong common elements. Both are short compared to other kdramas (only 12 episodes). Both are tearjerkers. Both male leads have an abusive father. Also the watch is a link between these two kdramas.

Both Drama have an intense Love Story, a beautiful relationship where they would both sacrifice for each other. Both shows are must-watch and they have an extremely memorable ost. They will make you feel every kind of emotions.

Both are heavy political dramas and have a Crown Prince as ML. Both crown princes are very similar and want to be good kings who really care for people. Haechi has more action while Secret Door is more psychological. One thing to keep in mind is that the 2nd male lead in Secret Door is also the main character in Haechi. Both characters and dramas are based on true story. Obviously each drama tells its own version about personality of this prince (and later King).

They are set in different times and countries but they have some similarities. They both have the "indipendence" element against Japanese colonialization (in BoC more). Both have great (and mature ) romance and respect between the leads. They are VERY different but I feel like many people who watched one or vice versa will be satisfied. Both are great dramas and have great characters which you will be connected with.

Maybe it was the "smallpox" vaccine's presence or the fact that both have the same "foreign" actor (and theme) and a great female lead, that I recommend watching both kdramas. Of course, Rookie Historian is much lighter and has "young actors" but Jejoongwon is so worth the time you spend. It's much more mature and makes you feel a lot of emotions. Both want to break "prejudices" and discrimination against a certain group of people: women and foreigners in Rookie Historian and "butchers" or"untouchables (baekjong) , women and foreigners in Jejoongwon.

Even though, they are fairly different (one is a cdrama, wuxia with fantastical elements and Haechi is a sageuk), the male lead has a "similar" treatment. Many times he is misjudged but has really good intentions. Maybe if you liked The untamed, Give a try to this one, because you will meet many characters and every one will remain in your heart. Also bromance.

Similar vibes, slow and "sad". Also the same "accident". In "chocolate" characters were not a couple before as in "when my love blooms" but there are also time gaps. Both are melodrama. Both have flashbacks.

Of course, one is historical and the other one not, but I feel like they have similar vibes. Both expecially have STRONG female leads who speaks always about "workplace" abuse.
Both recommended.

Didn't watch Nine but I feel like both will be similar in terms of time travel and stopping crimes from happening. "Boku Dake ga Inai Machi" was amazing and it is derived from a manga.

Watch it if you love dramas about time travel and particularly stopping crimes from happening. Both are amazing and will leave you craving for more. Smart male leads.

Main male leads have a similar past, and they are both known under an ugly name connected to their past. Investigative, murders, torture and disturbing cases.

Spoiler:
Both have kind a strange ending (much more Voice 3 but still.)
Similar vibes, overall