Amuro Ray

They are not exactly what you look for, but, in the manga I know, these can somehow fit the description:

There's Sidooh. I think it's the closest to what you are looking for. The protagonists starts when they are very young kids and we see the struggle they face as they grow older. They are accepted in a group after being alone for a while. There's slight romance, but it's late in the story and, honestly, don't expect much coverage about it. It's dark, drawing-wise and story-wise. The story takes in Japan and starts at year 1855. It's a historical manga.

Vindland Saga is similar to what you demand. The protagonist starts as a kid on a quest for vengeance and blood. Then, later, he is accepted into a group. It's also set in a medieval like context like Berserk. Vindland Saga is more historically based though.

Shut Hell don't tell the story from a very young age (well, for Yurul only) but it's similar to a certain point even if the group is pretty small. The setting is also pretty dark, but not the drawing. The setting is in also in a Medieval setting. This time, it's in Mongolia.

I think Shamo also fall in that category. Ryuushima starts as a teenager and the story is pretty dark. He is not really accepted into a group even though he gets allies. This time, it's in a modern Japan setting.

All of them don't really have romance. No more comes to mind.