Why do you want to use a PCA9685 to operate the servo's? If you need a Arduino to connect to LocoNet, than you can also use this Arduino to operate the servo directly. I2C is much more limited regarding the wire length than LocoNet. So place one Arduino near a couple of servos and connect it to Loconet and the servos directly. If you want to use the mrrwa/LocoNet library, you need an Arduino with two 16-bit timers, e.g. a cheap and small pro micro.
To easily move the servos slowly — e.g., for turnouts or semaphore signals — you can use my MobaTools library.