You need to perform offset calibration, otherwise you'll get large drift errors. The MPU6050 has a dedicated Digital Motion Processor (DMP) where data from the gyro and accelerometer are fused for you. I recommend grabbing the results of that fusion from the DMP. As far as I know the only libraries that do this are ones based on Jeff Rowberg's work. For a ...
With 8 TCA9548A 1-to-8 I2C Multiplexers on different I2C addresses, it is a multiplication problem: You can expand the Arduino Mega's I2C port on Digital Pins 20&21 to 8 multiplexers * 8 multiplexer ports * 8 ADS1115 addresses to 256.
It might be possible to go much further if you can wire and code multiple levels of multiplexers.