It appears that the British Microlight Aircraft Association is who you should refer to. They have a document on Differences Training. Details of all differences training can be found therein, but some of your specific questions pulled out...
Autopilot is mandatory:
Before exercising the privileges of the rating, the holder must complete appropriate differences training, if the
aeroplane is fitted with Autopilot System
and the holder does not have training or experience in aeroplanes with such fitted features. The differences training
mentioned above must be given by a flight instructor entitled to instruct on the aeroplane on which the training is being
given, recorded in the holder’s personal flying logbook and endorsed and signed by the instructor conducting the
training.
Retractable u/c is "advised" (Egad! This is the one you don't want to get wrong!):
Although not specified in the August 2021 ANO update as requiring differences training by law, landing with the
undercarriage retracted is one of the most common accidents in complex light aircraft. Light aircraft training typically
includes a “gear down” check as standard in circuit and before landing checks. All precautions should be taken to avoid
this sort of accident; several broken aircraft each year demonstrate the importance of this training.
Variable pitch prop is mandatory:
Before exercising the privileges of the rating, the holder must complete appropriate differences training, if the
aeroplane is fitted with a variable pitch propeller
and the holder does not have training or experience in aeroplanes with such fitted features.
The differences training mentioned above must be given by a flight instructor entitled to instruct on the aeroplane on
which the training is being given, recorded in the holder’s personal flying logbook and endorsed and signed by the
instructor conducting the training.
Comment: For clarification, a variable pitch propellor (VPP) is one whose angle of attack can be adjusted in flight. Many
microlights have ground adjustable propellors whose angle of attack can only be adjusted on the ground; these are
not included within this differences training requirement.
Mishandling of a VPP will likely lead to engine damage with the safety risk that it brings and to extremely expensive
repair bills. The differences training will consist of ground training to understand the workings of such devices and
flight training to practice the practical use of such systems, failure modes and emergency procedures.
Is there one "course" which covers all differences you might be interested in is probably a general "no", however if you have access to such an aircraft which has all the features you're interested in and an instructor/examiner qualified to carry out such differences training I'm pretty sure they would do them all in one go. Not a microlight, but that's how I added retractable & complex (basically: Constant Speed Prop) endorsements to my SEP rating on my PPL.
