Basically I'm sick of the usual teenager in another world crap, I recently read GATE and loved the idea of a adult going into another world but not being completely obsessed with magic or going on some hero's quest.

I personally would like a story with a selffish/morally ambiguous MC who has more or less not inclined to do anything heroic unless fully compensated.

Doesnt have to be scumbag but clearly isn't a bleeding heart or horny moron

Exterminator is an exterminator going to another world to keep being an exterminator, or Risou no Himo Seikatsu is an office guy going to another world to be a neet.

Seconding Exterminator

Dungeon Nursery and Meikyuu Black Company both have adult MCs who are kind of assholes.

Not quite an adult, but the main character in Ore to Kawazu-san no Isekai Hourouki is a university student, and he basically just spends his time messing around and using his incredible powers to do whatever he feels like.

And, despite being about a teenaged MC, Wortenia Senki basically hits all your other points. The protagonist is super mature, morally ambiguous, and really not inclined to do any "saving the world" stuff unless properly compensated. Isekai Mahou wa Okureteru! is also a bit like this, with an OP protagonist who just wants to go home.

Otherwise...

Takarakuji de 40-oku Atatta n dakedo Isekai ni Ijuu Suru stars an adult protagonist who wins the lottery and starts bringing modern conveniences to another world that he happens to stumble upon. I personally find it kind of boring, but YMMV, I guess.

Tondemo Skill de Isekai Hourou Meshi is basically a cooking manga set in another world with an adult protagonist.

Potion-danomi de Ikinobimasu! has an adult protagonist who "reincarnates" into a younger body. The story so far just involves her trying to find a good and easy way to live in the new world. I like how OP she is, but there's really no plot to speak of.

Koushaku Reijou no Tashinami's reincarnated protagonist uses her previous skills as a tax officer to basically reform the tax system in her family's fief in the other world. So basically one of those "kingdom-building" isekais but with a female protagonist.

Isekai Yakkyoku has a previously adult protagonist who also reincarnates into a younger body. A medical drama set in another world, basically.

Isekai Houtei: Rebuttal Barrister's protagonist is a washed-out, not-quite barrister who has to make his living in the other world by actually being a good lawyer. The author is usually a psychological manga writer (he wrote Kakegurui), so this one's very different from other isekais.

Isekai Omotenashi Gohan is another cooking manga set in another world. The main character is the older sister of the "chosen one", basically, and she cooks for her younger sister.

Death March kara Hajimaru Isekai Kyousoukyoku is also one of those with an adult protagonist in a younger body. This is kind of a generic isekai, except with an adult (who acts like it). I personally found it kind of boring after a while, but again YMMV.