This says that sometimes younger men can get RSS - https://www.dermnetnz.org/topics/male-genital-dysaesthesia/
However, it sounds like they're treating you for it anyway, just without the diagnosis, if they are following the treatment protocol in the above site.
Amitriptyline is super sedating. I was on it once for chronic pain, and couldn't function until 2pm the next afternoon. You aren't alone in not being able to take it.
All the sites I saw with a quick Google say to eliminate soap and use a non-soap cleanser, so definitely start there.
Also, think of every single thing that comes into contact with the area. Laundry detergents, body washes, clothing, wipes, underwear., etc., all touch your skin, and they can all cause allergies or reactions. Laundry detergent should be free of scents and dyes. If you use moist wipes in the bathroom, make sure they are unscented, as should your toilet paper.
Your underwear should be 100% cotton and breathable, and you should change it every day. (I just saw a study that says 45% of Americans wear their underwear 2 days or longer, so now I'm telling everyone. - https://tommyjohn.com/blogs/news/underwear-hygiene-study)
There are other treatments listed in the first link I gave you. Maybe you should see a urologist since the GP and derm didn't do too much.
Let us know what happens.