Really your situation seems a bit similar with the exception I have Gleason 9 and post HIFU and have other health conditions (post polio which causes fatigue a bit like ME but also I start with less muscle and use wheelchair sometimes)
I have been on Bicalutimide for 6 weeks now and about to have first injection of Zoladex next week prior to radiation therapy - monthly to monitor and adjust to check how I cope given my conditions. The post radiotherapy Injections or Bicalutimide we will decide at the time depending on the radiotherapy outcome and PSA nadir but up to 2 years - we have taken a course of action that monitors my particular side effects and reaction and gives flexibility as and when / hence the use of monthly rather than 12 injections.
i am quite apprehensive about the Zoladex as so many write how bad it is. Whilst fore 6 weeks of Bicalutimide has had almost no side effects only very minor nipple itching my PSA dropped to from 6 to 1.8 after 5 weeks.
so I think your path maybe similar given the Co morbidities.
i wonder how quick or severe the reported effects of the injection will be though!?