This issue is discussed in the following article. It includes the UMich study (which was not a randomized trial) as well as several others./pcnrv.blogspot.com/2016/08/combining-androgen-deprivation-therapy.html
As you see, there are no standard answers. Your pathology and your initial PSA with its doubling time have to be considered.
18 months is not "optimum" - we have no idea what optimum is, and it will likely be different depending on patient disease characteristics. 18 months came up in a study by Nabid et al. that showed that outcomes of primary
radiation therapy were no different with 18 vs 36 months of Casodex
(not Lupron). It does not apply to your case. However, one can conjecture that if 18 months of Casodex was no worse when used for primary radiation, maybe 18 months of Lupron is no worse when used with adjuvant/salvage radiation. It sounds plausible to me, but arguments for not applying it sound plausible to me too.
I think you have to go with whatever makes you feel most comfortable. You used bold type in your signature to highlight your T3b stage and Gleason score, and you are right that those are troubling risk factors. So are your high and rising initial PSAs. Based on those, and if the side effects are tolerable, I think you are justified in wanting a longer course of adjuvant ADT, and two years is certainly within what many ROs would consider to be reasonable.