Encoders only report steps from starting position which will be unknown so you cannot output an angle, hence the libraries only report steps. If you know the steps the conversion to angle is trivial - see below
The Index pin appears to mark a zero point.
So your code will most likely involve pretty much any of the libraries, possibly modified to turn off pullups
Starting the encoder processing
Setting up to monitor the index output on a pin, preferably with an interrupt
Waiting until the index pin triggers (encoder will need to be moved through full range by whatever it is connected to)
Zero the encoder count
From there your angle is (encoder count%2048)*.176 though check that is actually the case once you have the hardware and work out how to handle your rounding errors (otherwise you may have oddities going from 359 to 0)
Given encoders can miss steps for various reasons if this is mission critical you will also need to keep monitoring index and handling the situation where it triggers when not at zero, simple resetting of the count to zero will work but may have unpredictable results on down stream equipment so think about what happens in this case.
You also need to work out how the 'rotate full cycle to find index' works in your setup process without having side effects.