I think a timer in a collectathon wouldn't be a good idea personally. If the timer resets the level while the player is exploring it may break the immersion. Maybe timed events for reaching a collectable item would be a better approach?

Looks very nice! But the chest in the 2nd screen shot looks a bit jarring facing the camera from the side like that IMO. I'm not sure if it would looking better just facing forward or in a 3D style like the house. 🤔