hello! if you're playing on pc, theres a walkthrough .txt in the game files that will (hopefully) help you out. (its a little barebones as of right now, i do intend to add better descriptions to it some point soon.)
if not, any extra content is all before lavandula's ending dialogue. The ending you're looking for actually happens pre-basement level, though theres some alt CGs that can happen after that.
So as a hint: be sure to exhaust all of miriam's "question" dialogue and check both floors for any hallway choices that unlock thereafter. Also, pay attention when youre travelling down (either) dark hallway!
thank you for playing, and reaching out! :D