Considering this is supposed to be the full game, with no intention to add anything further except the split-screen co-op and perhaps more bug fixes, the winter lantern arrival seems about right on point considering. If this were much more of a "full-length" game the natural arrival of the Deep Night scenario where the winter lantern arrives I think would/should happen after several areas and boss fights' worth for a last third or fourth of the game.
Also, I didn't know you kill the winter lantern one-on-one. I think the intention was for it be a "Nemesis" type enemy like the PS1 version of RE3, where you can technically kill off Nemesis whenever but it's neither worth it and uses up time, resources and a lot of risk. You're supposed to just run away until the end, where the way you are meant to kill it is with the exploding chandelier drop on it in Gilbert's Estate (which is also optional since you don't need to kill it to finish the demo unlike with Nemesis).
One thing I wish is that the "true" "secret" ending which requires maximum insight collected (20 total possible) is a split second flash you can't make out without pausing a YT video (in the distance you see Amygdalas and a few other enemies I can't make out).