Question about Final Singularity

Well. If you play it as spoil-free before your final involvement into that Solomon chapter, then I can say final chapter's story will surprise and amaze you. '

Also although raid quest was finished, you will still be beneficial from your friend servants' support to get through the main quest. Bond point level still work I believe. Equip right CE (like Origin Bullet) then you will fight easier in the boss fight. Normally speaking, the story in solomon chapter is more attractive than gameplay of boss fight.

The raid quests were basically a way for you to milk the bosses for mats, but otherwise, no, you didn't exactly need to participate. All I can say is that if you're going into the story blind, then that would be for the best. This chapter is spoiler-heavy, but you do have to watch out for the big boss fight. The guides should help you, but if you manage to make it here, that means you had to get through both Camelot and Babylonia, and that alone shows Solomon should not be too overly hard of a challenge if you did, especially if your servants are at a high bond level.
Also, Shinjuku should come out soon, so hopefully you will be able to catch up quickly enough to get on the bandwagon.