Welcome to our Forum. I'll try to help. Please remember that no single exposure to a partner of unknown partner has even a 1% chance of leading to HIV. Further, as you point out, you already had a definitive test result before you took your steroid shot. There is no reason to continue to worry about HIV.
As for your specific questions:
1. What is the chance that my single steroid injection would affect my 8 week test took 21 days after the shot?
Zero.
2. I know that this is too late for ARS but would dermatitis be a symtomp of a immunosuppresion? Could I be immunosuppresed 9 weeks after the encounter?
No. Steroids themselves can cause acne but what you describe, in location and description sounds more like folliculitis than anything else.
3. Should I consider myself safe or another test is guaranteed?
You should believe the results you have. There is no need for further testing.
4. Does this (bumps in belly and dermatitis on beard) seem like another STD?
No
I hope these comments are helpful. Take care. EWH
Thank you Dr. Last question. Today i started to develop a sore throat. So far no other symptom. Would it be possible to start having it as a symptom 9 weeks later? Any other test would be useless?
Your sore thraot has occurred too late for this to be a manifestation of recently acquired HIV. I recommend that you believe your test results. There is no need for further testing related to the events you have described. EWH