Is it possible to command an autopilot to start a fixed rate descent? For example to avoid weather during the cruise phase? From what I have read the autopilot would maintain constant speed and altitude until interrupted but havent found a fixed rate descent function.
Here is what typical autopilot controls look like
Image source: What is a "Flight Director"?
The pilot can set the required altitude, as well as the required vertical speed to get to that altitude. A cruise decent will typically use a shallower descent than an approach descent.
This answer explains that there is a flight level change mode and a vertical speed mode. The latter will hold vertical speed until a target altitude is reached.