Nomis, it sounds to me your situation is highly unusual and one where the oncologists are feeling their way and perhaps finding themselves a bit on the back foot over giving answers to questions where there is little definite knowledge.
My presumption is that the concern is that anyone on any sort of testosterone reduction is likely to suffer muscle loss, which is likely to be more serious and more difficult to counter in your case. (I have been on Zoladex recently, and hoping that weight-bearing exercises and vitamin D supplementation will stop me having problems). But doctors know that radiotherapy is more effective when the cancer cell growth is inhibited by removing testosterone, and with your high Gleason score they think that gives you the best prospects.
From my understanding (I am no expert, just a knowledgeable patient!) I would be exploring whether it would be feasible to just use bicalutimide assuming a few more weeks gets your PSA down even more. You don't mention having bad side effects so far. Zoladex is more effective but also much slower to reverse, so would probably affect the muscle more in total than bicalutimide. However I am assuming you could come off the drug not long after finishing radiotherapy, and probably standard protocols for your Gleason level would normally keep a patient on reduced testosterone for longer. But one way or another they are going to have to decide on what to do, you are not a "standard" patient and there won't be clinical data on similar patients to guide them, and the underlying biological processes suggest that once radiotherapy is finished the benefit of maintaining low testosterone may be less clear.
But back to your original question of how people have found treatment with Zoladex: for me it wasn't that bad (not that I would have had it from choice!) Stamina was reduced somewhat - at the moment I struggle with late nights - and there is less "get up and go". Plus obviously effects on libido, and according to my wife a change in body odour. But in practice it hasn't stopped me doing any of the activities I normally do. I have only been doing 15 minutes or so of specific exercise, though I have quite deliberately been trying to keep up general activity levels (aiming for a target 10,000 steps a day, or an equivalent cycling). On the other hand my last dose theoretically ran out two months ago, but its effects continue as far as I can tell.
However ... few others have your combination of prostate cancer on an existing condition, and the optimum treatment is always going to be an informed guess. Ultimately you will have to collaborate with your oncologist to decide on something they think is likely to be effective and you are happy with in terms of adverse impact specific to you. There won't be a known "best" approach.
My best wishes, it is not an easy time facing these problems.