Did you check if that same 6am slot works on weekends too? I feel like people's morning scroll habits change a lot on Saturdays and Sundays, so the magic window might shift by a couple hours. Might be worth testing both to see if it's really about the time or just about catching people before their day starts.

A buddy of mine went through this exact thing in his Wash Park place, same era. He DIY'd the fire blocking at first and it turned into a mess because you have to cut blocks for every single stud bay between floors. We're talking dozens of little pieces, and each one has to fit tight or it does nothing. He ended up hiring a guy after two weekends of that, and the guy knocked it out in a day, though he did have to open up some ceiling to get at it. Cost him around $1,500 but he said it was worth it just for the drafts going away, let alone the safety part.