Guessing the limitation is due to the Su-35's radar size and performance relative to the Zaslon and lower top speed for a launch?
Well, it's still a 45t bus with largest and most powerful fighter radar,
Not sure about this tho.
Current Zaslon AM has only about 12.5 KW peak power and 2.5 KW Average, vs Irbis 20KW peak and 4 KW Average. Also Irbis comes from much later generation of radar's. Unlike Zaslon which does not have receiver stage within its antenna, Irbis has it like Bars. Thus it allows low noise receiver 3.5 dB vs 5.5 dB for Zaslon.
Zaslon is unfortunately running out already of its upgrade potential at the moment. Given that the Zaslon-M Was never produced, MiG-31BM relies on Zaslon AM retaining the original 8B antenna but newer processing.
Kinematic wise tho i dont see why Su-35 can't make full use of R-37M. The weaknesses over MiG-31 is perhaps it lacks WSO, thus there could be workload issue for long CAP mission.