My assumption [was] that because an engorged penis is a numb penis, the soreness was due to insenitivity during engorgement to abrasion, bending, etc. In any case I definitely experienced soreness in the hours afterward as well as sometimes longer term. I was using a much lower dosage, so I don't think it is dosage dependent, although perhaps a less hard penis translates to less soreness later. I tailored dosage, as I understood to be the goal, to be sufficient for intimacy but not too much extra. I tracked this by noting time to 1/2 engorgement. If time to 1/2 engorgement was greater than 90 minutes, I took ephenedrine and lowered the subsequent dose. If it was under 45 minutes, I increased subsequent dosage.
While it is true alprostadil (I think that's the one of the tri) deteriorates, it's about 20% in 6 months under consistent refrigeration, so that can be adjusted for fairly easily, and good needle and storage hygiene left me with little concern about contamination so I felt comfortable ignoring the compounding pharmacy's "1 month" shelf life.
Ultimately Vacuum erection device (pump) was also helpful, I was finally able to develop tolerance to the PDE5 inhibitors, and my wife objected to the erection without stimulation, as it felt "urgent" and artificial, so we have left the trimix life behind for a softer, but more satisfying approach. (Marginally enough engorgement for some penetration.) I'm thankful trimix was an option and I will use it again if we feel it would be our best option, but I'm okay with where we are now.
A key element for me is that there is no direct correlation (in my case and based on what I read) between engorgement and orgasm, in spite of my many decades of experiential association.