https://www.mangago.me/read-manga/i_want_to_be_the_wall/
The main girl and guy aren't involved romantically together but romance is a big theme. I highly recommend!
May bad idk. Similar one tho, 20th Century Boys. Friendship's a big theme and almost no romance. But it's mainly a guy friend group with 1 girl in it, also everyone and their grandma have probably read it lol. https://www.mangago.me/read-manga/20th_century_boys/
https://www.mangago.me/read-manga/off_kai_shitara_tondemonai_yatsu_ga_kita_hanashi/
This one’s really cute if you want to try it!

Something where the main plot is friendship between adults of opposite sexes? No romance, just good ol' friendship, overcoming the hurdles in life with each other's help like good friends do, etc.