Hello Jimmy, sorry to hear what you are facing.
I haven't tried erections with injections, so I'll leave others to say how they feel.
You don't need any arousal for injections to work - they don't rely on any nerve signals from the brain. Equally, they don't go away when you get bored - they go away when the drug eventually disperses away. You don't need any nerves for the injections to work - not sure what your urologist meant yesterday. You do need a good penile blood supply though, and this is a common reason men get erectile dysfunction as they get older. If your erectile function is good now, then that's not a problem for you at the moment. Is your surgery likely to interfere with this?
There is a cream (Vitaros) or a pellet (MUSE) you put up the urethra, but they don't work as well as the injections, so they don't work for all men.
There are two different drugs used in injections - some people find one is better than the other for them. There are different types of injection syringes too. You would go to an ED clinic or Andrology department, where they will go through the treatments.
Many men can orgasm without an erection, but it's more difficult. There are male vibrators which might help with this too. You can try a pump too, and you should use a pump regularly anyway, because you won't be getting natural nightly erections, and your penis does require regular erections to maintain health, particularly of the erectile tissue.
Another option particularly at your young age would be a penile implant, where the erectile parts of the penis are replaced with inflatable tubes/balloons, and you can create an erection whenever you like by pumping them up.
I suspect the procedure you are referring to to lengthen the penis is cutting the ligament which makes it point up. This would be an unusual case so if you were thinking of going this route, I would suggest talking with a very experienced penile surgeon to find if it's likely to make any difference in this case. Prostatectomy patients sometimes find regular use of the pump significantly recovers from penile reduction.