[Spoiler, I think...] Question about Final Singularity

I'm not too sure how it works myself, as I wasn't around for Onigashima. But I recently rushed through the singularity in JP in order to participate in the Christmas challenge quest and the bosses were only one battle each. So I assume after the "world" defeats the raid boss it stays defeated. Also the singularity itself closes after you complete it so it sounds like it's similar to an event in that you farm as much as you can, while you can, before everyone collectively defeats the pillars.

Solomon chapter will close after you beat the last boss. You can check https://fategrandorder.fandom.com/wiki/Salomon. It's stated singulaity will be locked after its completed.