Reading this on other sites, it has ended like a LN advertisement at chapter 111. Development was alright but ending abrupt.
Not exactly right. They released 112 in two parts in April, Comikey flags 112.2 as END (and paywalled it), volume 12 has (almost) the whole harem in the cover, and from what I can see at Era Sunday, you are right that they ended it as if it was a LN ad, so it either got axed or it caught up to the source material.
EDIT:
@Sollertia_ the weird thing is that I just went to check the status of the LN and it's been stalled since a couple of years ago at the daughter arc. And to make things worse, while the related WN about the sage is posted online (and finished), this one seems to be an original LN, so no luck there either. In fact, he seems to have stopped all of his WN activity in 2019 (leaving several series incomplete, included published ones), and most of his LN activity in 2020, with this series going dark in 2021 and
The strongest assassin reborn as an aristocrat in another world (the last one he seems to have worked on) in mid 2022.