Maybe the best strategy is a single non-pro model. Both the mini and the plus seem to be unpopular when the Pros get all the new features.
The 3-week delay in its release probably doesn’t help either. It would be as if the iPhone 8 came out 3 weeks after the X.
The biggest problem is that apple is missing the correct demographic, people who wanted the mini where not the people who wanted the cheappest iphone, there is the SE for that, it's people that really wanted the form factor but with all the advantages of the pro even if at a premium, if the mini was launched as the pro mini, instead of default mini, sales probably would be better, same deal with the plus, for a little more you get an device with an already big enough screen but with better cameras (which people care the most about nowadays) and more shiny new features.