What is the point of no return?

[raxomachine]

I've already got the 3 souls but there is more I can explore but I don't know if going back to the egg will lock me out of additional content and I saw another post saying something about having to restart to play the new expansion.

[Miri Ryllis ミリス]

Absolutely nothing is locked out from you going to the black egg at this point in time. Do note that there are 3 endings to this game, 1 of which gets locked out if you explore/progress too much into the story. You don't need to worry about that right now. The point of no return, without saying too much, is when you join the halves and dream in darkness. . (tagging JIC you dont want to see it). If you choose to go there now and finishing, reloading that save will throw you to just before the room, a-la Zelda games.

As for restarting to play the Grimm Troupe, you do not need to. It is available to you right now if you so choose to seek it out. What you may have to restart for is seeing both paths the DLC has, as you may only choose 1. Or just, you know, make a duplicate of your save file before the decision point and do it that way