If statements with multiple null conditions

Hi,

I am slowly learning Power Query M, but I've gone down a rabbit hole and neeed some help to correct my thinking please! I need to return a column that provides a single date from multiple date fields, based on a heirarchy.

The plain language version is:

If the [assignment.scheduledate] date field is not empty, return this date, unless the [schedstart] field has a date and this date is greater than the [assignment.scheduledate].

If the [assigment.schedule] date is not empty then return the [schedstart] date field. 

If the assignment.schedule date and the schedstart date fields are both empty, return the targstartdate field.

I've tried many variations of the below, but cannot seem to get an error in the results when there is an [assignment.scheduledate] . Any help would be very much appreciated!

if [assignment.scheduledate] <> null and ([assignment.scheduledate] >= [shedstart]) then [assignment.scheduledate] else
if [assignment.scheduledate] <> null and ([assignment.scheduledate] < [shedstart]) then [schedstart] else
if [assignment.scheduledate] is null and [schedstart] <> null then [schedstart] else
if [assignment.scheduledate] is null and [schedstart] is null then [targstartdate] else null)