I guess we’ll know in September
Just kidding.
There was something similar between iPhone 12 Pro and Pro Max. But the 47 %!!! larger sensor of the iPhone 12 Pro Max was almost unnoticeable. You will find many comparisons, hard to tell a difference. So maybe Apple tries to keep the differences artificially smaller, that would make it less terrible to go with the normal pro.
But to answer your questions:
Will only the Pro Max receive the newer and bigger sensor? - Well it happened before and yes I believe Apple could do it again. The rumors indicate it, so probably that is true, unfortunately.
300 mm Superzoom camera - There wasn’t a lot of chatter about it. I doubt it as it would only make sense in two unlikely scenarios. Either with a real variable zoom, or as a fourth camera. I don’t know how much 300 mm
would be in times x zoom, but there is no point in having something like 1x, 2x, 12x. The gap is too big. So either it’s a variable 5-12x lens, or super zoom is coming as an additional camera. But all of this is speculation. There are no rumors regarding a fourth camera and no rumors regarding variable zoom I think. Maybe the current 5x was already Apples superzoom?
Edit: With a 48MP Telephoto sensor and 5x lens next year Apple will possibly introduce a 2x crop mode similar to the current wide camera, where just the inner area of the sensor is used. That will create a kind of 10x zoom level providing 12MP photos. So the iPhone 17 will likely have this feature.