I'm looking for a romance manga where the two leads are good friends first. Fullmetal Alchemist, 19 Tian, and Tamen de Gushi come to mind as good examples of this.

Since your examples had 1 each of romance, shounen-ai and shoujo-ai, I'm going to rec from all three of those genres. Hope that's okay.

BL and GL:
Komatta Toki ni wa Hoshi ni Kike! -- The ultimate example, at least in the BL genre.
Bokutachi Otokonoko
Girl Friends
Itsuka Tomodachi ja Nakunaru to Shite mo
Sasameki Koto
Yes It's Me
Moonlight Flowers
Blue Wars
Donuts Tsuushin
Aoi Hana
Asterisk -- Though I must add that this was too boring for me

When it comes to romance, shounen is probably your best bet on this front, since it features the childhood friends --> adulthood lovers trope in abundance. I don't read much of those, but we'll see what I can come up with:

The Pale Horse
Katteni Kaizo
Kekkaishi
Kindaichi Shounen no Jikenbo and its many, many associated series.
Naruto, of course
Beelzebub
Ueki no Housoku
Montage (WATANABE Jun)
Saike Mata Shite mo
Merikuron no Namida
Spiral - Suiri no Kizuna
Utsuro no Hako to Zero no Maria (Novel)
Swweeet
20 Seiki Shounen -- Though it's not the focus of the story at all.

Stein's; Gate -- Best watch the anime for this.

And you might want to look through the Friends become lovers category for more.