I decided that I would give a P3D aircraft from CaptainSim a shot, and I am feeling massively let down again. The VNAV is nonsensical, it ignores altitude restrictions and constantly commands a -6000fpm descent. The approach mode on the autopilot is also broken, it doesn't show a glideslope or localizer on the display yet it says they're captured. After the aircraft said it had captured the glideslope, it still continued to command -1800fpm down, and I crashed into the ground ahead of the runway.

Definitely wishing I could ask for a refund, I have been burned by CaptainSim twice now on completely non-functional products and I will never be buying from them again.

After a few trial-and-error flights, I have found that doing climb and descent using vertical speed mode/LVL CHG (for the climb) works pretty well. I also found that the issue before was most likely a sim issue with wrong frequencies than it was the aircraft, as I was able to perform an ILS landing into KOKC as I expected it to work. There are still some weird small bugs, especially related to the LNAV and its pathing just after takeoff on some departures with rapid turning waypoints, but otherwise I have figured out how to operate the aircraft in a realistic manner.