To be clear, medium that is both phenol red and riboflavin free is what I found to be very expensive.
I’m not to familiar with long timelapses using light sheet. 12 ml for a single slip is a lot of medium, I’d like to think you could go quite long, is perfusion on option? Maybe not on a light sheet given some of the challenges it presents.
One thing to remember is the biology - there are cells that probably exhaust their media or acidify or release something else that is toxic, where other cells do not. 3 and 4 day time lapses are pretty common and these cells are often in 6 or
12 well plates. You can add ROS scavengers to medium to help address if effects are due to ROS production, an old school scampered is ascorbic acid, but I’m sure there are other, better options.
Best wishes on your experiments.