Found it:
https://www.google.com/maps/@35.3647979,-103.4108457,3a,70.9y,272.57h,95.9t/data=!3m6!1e1!3m4!1s-wOistBmc8SJc7HMJFf2nQ!2e0!7i13312!8i6656?hl=en
Just wanted to mention because many people here seems unaware of this: Butlers were nobles. They were most often second born or later sons of poorer nobles like gentry in England and as such they were usually landless, but they still were part of nobility.
Yofukashi no Uta seems more like short, story rich manga like 7 Billion Needles or Spirit Circle than never-ending shounen. I think we will get main story arc soon.
Isekai/reincarnation of human into a monster - Check
Absorbing defeated enemies skills - Check
Most powerful/weakest (but in reality strongest) monster - Check
Gigantic dungeon - Check
1st person narration - Check
And we have a Bingo!
Additional tags include:
Nonsensical female adventurer...