How to change default sort order in Catalina's Reminders?
I am inclined to accept the comment by @Joakim Danielson as the answer. Accordingly, instead of expecting a solution in a .plist file, I have written this Applescript to execute on startup (you also have to grant it accessibility permissions in Security &amp; Privacy)

tell application "Reminders"
    activate
end tell
#wait for Reminders to launch since can't control startup items order
do shell script "/bin/sleep 5"
tell application "System Events"
    tell process "Reminders"
        tell menu bar 1
            tell menu bar item "View"
                tell menu "View"
                    tell menu item "Sort By"
                        tell menu "Sort By"
                            click menu item "Due Date"
                        end tell
                    end tell
                end tell
            end tell
        end tell
    end tell
end tell