The word 'should' does not mean mandatory, it is only advisory
The word 'should' rather than 'must' is used in all AMCs since there may be a number of alternative AMCs from which you could choose in order to comply with the requirement; it does not mean you can choose to ignore the requirement. The reality is that there is currently only one AMC for this, so you 'must' comply.
a 3 (iv) expiry longer than 3 years: the applicant should again undergo the training required for the initial issue of the rating
The CAA have chosen to pragmatically interpret this as the content of the course (not the specified duration). As stated above, essentially this amounts to training as required to include revision of all elements from the course.