The Point of No Return is when burning the Sealing Tree at Church of the Bud (unmissable story event). This is after the Romina main boss. After this all NPCs will disappear and their questlines will fail, except Ymir/Jolán and Igon/Priestess which are the only ones that remain doable post-story. Do all questlines before burning the Sealing Tree. Keep a backup save if possible.