Actually I never watch long Japanese drama except Oshin (because it's on TV) and Gochisosan that I dropped. Oops.
I haven't watch many Japanese romance dramas, but since romance dramas are universally loved, maybe I'll recommend some not so new happy ending romance drama:
A Calm Sea and Beautiful Days with You ML (a navy officer who's alone in this world) and FL had an arranged marriage. Slowly they start knowing each other.
Hold My Hand at Twilight FL being dumped by her long distance boyfriend who promised to marry her so she had to earn money to build her grandma an elevator iirc that her boyfriend promised her as a dowry. FL and ML (an aspiring idol) live together later. They already met twice before living together iirc.
Nice Flight! ML was a co-pilot/first officer and FL was an air traffic controller. They can't be more different, except for their love for aviation. ML adored FL without knowing that it's FL so SFL pretended that she's the one ML adored.
Silent ML left without a words because he's suffering from hearing impairment so fast forward a few years now FL and SML (ML's bff) dating and living together, but then they all met again.
Why I Dress up for Love ML and FL can't be more different, so when they live together they started bickering about whose lifestyle was the right one (there's another 2 + 1 people who also live with them) but they also get attracted with each other.
I see that you also watch BL, I'll recommend a few titles:
Love Is a Poison One was a lawyer, one was a conman. As the conman helping his work, he agreed to let the conman live in his apartment.
Our Youth One of them was a model student, the other one was a delinquent who got beaten by his father. But by chance they became secret friends.
Perfect Propose MC was overworking by his company, one day a young man came to live in his apartment and taking care of his every needs.
And if you don't mind bad ending, you can try:
Eternal Yesterday One rainy morning, his best friend and secret crush died from getting hit by a car (white pick up maybe) and became a living death. They then kept living their life like usual but the clock was ticking.
The Diamond Sleeping in the Sea FL saw the man who looked like her first love so she went after him and decided to help him find his goal in life while ML help her find her first love. It went back and forth from present to past, with the actor playing ML in the present and ML in the past. The romance happened between ML and FL in the past.