• Dragon Quest: The Adventure of Dai (100 episodes)
• “Oshi no Ko” (35 episodes across three seasons and a fourth on the way)
• Kaguya-sama: Love Is War (37 episodes across three seasons and multiple mvies and specials)
• Deltora Quest (65 episodes)

Really hard to find English subtitles of Deltora Quest for the last several episodes that serve as the epilogue of the series due to it containing liscensed music.

Space Brothers still just has the one season, but its 99 episodes + a movie.

Frieren

Horimiya

Fullmetal Alchemist

Blue Box

My Dress-Up Darling

One Piece

Spy x Family

Jujutsu Kaisen

Hunter x Hunter (2011)

Oshi no Ko

Jigokuraku

What is your definition of long running? At least 20 Episodes? At least 50? At least 100??

Spice and Wolf

The Ancient Magus’ Bride

Log Horizon

Kakuriyo - Bed and Breakfast for spirits

Oshi No Ko?

Maybe try

-My Dress Up Darling

-The Dangers In My Heart

“Call of the Night” has two seasons, is relativly new and has romance without it being the main focus.

Monogatari series is quite long and is ecchi, but it is different from your “normal” anime so it is not for everyone. First seasons aired around 2015s if I’m not mistaken, but recent entries are around 2023ish. You need to look for watch order list to et the full expirience tho.

Two that come to mind, that probably would be a good fit for you

• ReZero
• Rascal Does Not Dream series (two seasons, three movies)

7 Deadly Sins

Quite a few seasons, plus movies. Fantasy. Includes mature themes, delivery is often humorous, but I wouldn’t say childish for sure.

Adding nothing of value here but I’m curious as to the responses you would receive.

The criteria of:

• modern
• long-running
• romance

turned out to be quite restrictive, and I can’t think of any good recommendation off of anime I’ve watched 😅