Hi Nigel, so you were on HT for about 6 months, then had RT. You are still having 6 monthly HT injections (presumably the most recent in January) and are due your next in July. With the prospect of being on HT until Dec 2025.
It is not so common to have HT for salvage RT as it is when RT is a primary treatment, however your oncologist has seen all your results so will know far more than anyone on this forum will. When RT is the primary treatment a total of two years HT is common, three years is not unknown, and occasionally only six months is recommended. There is research suggesting that 18 months is as much as is required.
Going with your oncologists recommendations is always the preferred course of action. They are trying to balance the best probability of cure with the least inconvenience from side effects. However they are thinking in terms of the average man with average side effects. None of us are average.
For an average man the probability of SRT working is 50/50, and the side effects are bearable. If the side effects for you are unbearable then you could stop HT early, but you will reduce your chances of a cure. It may go from a 50% chance to 45% or maybe to 10% I doubt anyone can answer that question, but you could try asking the oncologist.