I don't know why the phosphate remover is not getting you to 100 ppb. Hopefully the phosphate test kit is still accurate.

To the degree that there is free-floating algae getting killed by the ozonator, then that could lower chlorine demand, but supposedly the lower phosphate level and your still having chlorine in the water should be killing algae faster than it could grow so there shouldn't be a difference in chlorine demand. Bather load in the pool is low so that shouldn't create demand. If you were to get lots of pollen or other organics blown in the pool then an ozonator could oxidize that and have lower chlorine demand, but it takes a good deal of debris to show up as chlorine demand. So that just leaves loss from sunlight which should be a function of the FC and CYA levels.

Keep us posted, though a better experiment would probably be to have the ozonator off for a couple of weeks, then on for a couple of weeks, then back again. Doing a whole season comparison could vary if weather varies season-to-season. By doing something back-to-back in one season, it's less likely to have variation in weather exactly coinciding with your ozonator on vs. off timeframes. Of course, you'd have to settle in faster getting the SWG % ontime to maintain stable FC pretty quickly with the shorter timeframe experiment.