Sorry unrelated topic *about endgame*

When is the ideal time to make contact with the F. Flame? I’m hoping to do it as late as possible right before I’d get locked into that ending. Is there a clear “point of no return” where I should stop and go get the Frenzied Flame, or can I safely wait until after the final boss?

Please keep it as spoiler-free as possible. I’m mainly looking for timing advice, not story or lore wise. Thank you so much!