Perhaps Hana ni Arashi? It's short, at 2 volumes, but the heroine has super strength and a rich dude wants to marry her.

I strongly recommend Otoyomegatari. The art is beautiful and the story is relaxing to read.

So a strong, kind, beautiful and wise heroine. With romance. Okay.

Seconding:
Akatsuki no Yona
Siesta
Akagami no Shirayukihime
Otoyomegatari

Also:
Joou no Hana
Song of the Long March not exactly romance but it's still worth reading!
Kechonpa wouldn't call her gentle. She's kind of brash and um... hot-blooded(?).
Double Arts Kind of? Not physically strong on her own but she and Kiri are strong when they fight together?
Morgiana from Magi? Eventually. Once she becomes an ally.

There might be more... I'll add some later.

Um, Red River? Sounds like it fits almost perfectly with what you said, though not sure if it'll fit perfectly for your tastes. Honestly, it feels a bit different.

Also, 9 Banme no Musashi with a ridiculously strong girl, and not too much mental loopholes. Romance is there in one arc, but that same romance is probably the part I least liked... well, see for yourself.

Can't think of others right now. Gotta get my brain working...

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I honestly doubt whether the OP will be reading this. Will still give it a go though XD
Please Save My Earth - Mokuren fits
Miriam
Kemonotachi no Yoru
Sugar Dark - Umerareta Yami to Shoujo - not very sure about this
Blood Alone
Jin - The courtesan is not physically strong. But, she is strong.

Seconding -
Akatsuki no Yona, Otoyomegatari, Song of the Long March.


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I don't know if these are what you're looking for, but I suggest:

Samurai Deeper Kyo
Defense Devil
Kanata Kara - She's not necessary strong, but she's determine.
Rave
Meteor Methuselah
Otogimoyou Ayanishiki
Sora wa Akai Kawa no Hotori
Canon
Chou Dokyuu Shoujo 4946
Five
Gokusen
Kimi wa Boku no Toriko Nare

I don't know if they're what you're looking for, but I hope you enjoy them!