Step climbs - enter the initial altitude, not the final. If you have import step climbs enabled, the plane will import them from Simbrief into the flight plan and climb automatically at those points. Otherwise you can individually add the step alts on one of the flight plan pages.
Managed vs open climb/descent - the PFD display will display OP CLB or OP DES when selected (arrow down) and CLB or DES in managed mode (up arrow). The arrow up/down represent pushing or pulling the knobs, not a selected direction for the plane to go.
350 does not automatically descend, you have to put it into managed or open descent yourself. Recommendation is to set the lower altitude then initiate descent about 5 miles before TOD which will put the plane in a 1000 ft/min descent until it catches up to the descent profile when it will increase the descent rate to maintain the necessary profile.
Don’t know about the RETARD issue. Maybe check the Discord to see if anyone else has the issue.
