The FMC in the stock A320 is very basic at best. It offers only the most rudimentary functionality. And there’s zero documentation on how to use it.
The FlyByWire team have made the free A320Nx mod which increases the realism of the A320 significantly, including adding a lot of functionality to the FMC and to other “inop” functions in the cockpit of the stock plane.
I don’t really fly airliners, so I can’t give you specifics, but if that’s your plane, then you’d probably benefit from this mod significantly.
https://forums.flightsimulator.com/t/flybywire-a320nx-v0-4-0-released/302015?u=crunchmeister71