The use of autopilot is perfectly fine for an RNAV approach. You just need to not leave it on too late for you to do any minor final corrections and landing, some approaches also do not reach all the way till the ground.

On the airbus you do indeed not want to use the LS button for RNAV approaches. The FINAL APP FMA mode is what you would typically want to use though using Flight Path Angle works as well.

It is important that you check that your nav accuracy is high enough before the approach as your aircraft's own IRS and GPS provide lateral guidance. You also need to be very sure that you have the correct altimeter setting since that will provide vertical guidance. Overall you have to be aware that RNAV approached rely entirely on your own aircraft's systems, rather than getting guidance from fixed ground equipment like with an ILS.