@rkkn - Elizabeth Moon's Deed of Paksenarrion trilogy has a protagonist that starts out as a young adult recruit in a mercenary company. It's one of my two favorite series about Paladins and is much more realistic than LNs.
For LNs, The Saint's Magic Power is Omnipotent isn't that heavy on the adventure but the main character is a summoned adult office lady. If the slower pace doesn't bother you it's a nice change from magic high school and villainess novels.
I've Been Killing Slimes for 300 Years is cute slow life with an office lady reincarnated as an adult.
Probably too young to fit, but Saving 80,000 Gold in Another World's Mitsuha is at least a high school graduate and the series has her acting as an adult not going to isekai Hogwarts. If you buy this one be sure to get the Kodansha light novel volumes not Sol Press and not the manga.