yandere sub plot not yandere final boss. like I said it's poison but I was giving it the benefit of the doubt. I won't ever again.
The yandere subplot leads to the final boss, it's not the final boss itself. Or at least, it's not supposed to be. Looking around at the WN, mostly just reading the chapter titles not the actual content, seems like a lot of story got skipped in order to rush to the final arc with the doom tribe finding Mea and Giselle finding Abel. This leads to the actual final boss, the sealed god.
If you want to compare this manga to the LN that this should be based on rather than the WN, this isn't even in the LN yet so it's WN only anyway. The manga seems to have skipped LN 4 and 5, going from LN 3 straight to WN final arc. The LN seems to have been axed with 5 being released in 2018 and nothing since.
All in all it looks like it ended up this way because they wanted to give it something closer to an ending, rather than going through LN 4 and just stopping there. Not enough chapters to beat the sealed god as well and probably missing a lot of important plot points for that in the skipped content anyway.