The manga Koukou Debut is some 50+ chapters long, but the main couple is already together by chapter 10. Thus the rest of the series is about their relationship with each other and various other things, rather than most of the plot being about how they finally do end up together,

what other series (shounen is also fine) have romances in which the couple gets together fairly quickly, so that the main thrust of the romance storyline is NOT how they overcome all sorts of misunderstandings etc in order to start dating?

(No yaoi, yuri, ecchi, or smut, please)

Kareshi Kanojo no Jijou fits perfectly!

There is Otoyomegatari which would fit if not for the fact that the story shifts to other characters later on. I'd love if it return to the main duo, but I'm not sure if it ever will.

Dengeki Daisy - the main duo gets together relatively quick, but they reach only a certain level in their relationship and don't develop much further. Nonetheless it's still a pleasant read, I mean those two are even happy sitting next to each other. Oh and so far there were NO third wheels.

This is exactly the issue I have with so much shoujo that got me into reading mostly yaoi instead.

Anyways, these should fit - the together-ship isn't always super early, but they go on considerably afterwards:
Usotsuki Lily
Ai Hime - Ai to Himegoto
Oyayubi kara Romance
Sarashi Asobi
Black Bird
Get Love!!
Kimi ni Todoke
Ai Ore!
Mars
Kiss
Suki-tte Ii na yo
Hakuji
14-sai no Koi
Himitsu no Ai-chan
Ookami Shoujo to Kuro Ouji
Ojousama wa Oyomesama
Goong
V.B. Rose

And if you want something classic: Chizumi & Fujiomi

Hatsukare, I haven't read it all but it fits, since they get together in the first chapter.

W Juliet - they get together in like the first chapter

Bokura ga Ita fits. The story is about their relationship itself.

Ganbare Seishuu! Ganbare Seishuu!