Hi and welcome,
Your situation sounds, quite honestly, like you are endlessly stuck between several immoveable objects, and several irresistible forces! You have my sympathy.
Steroid induced diabetes is very unlikely to be something that you will be able to control with diet. Oh, avoiding carbs (potatoes, rice, pasta, bread and cutting out the sweet fruit, etc) will help. But it won't do all the necessary work. Are your numbers high/erratic all the time, or is it worse just after the injections? If so, then you may need to consult with your nurse, doc or specialist to work out different regimes for different phases of the steroid injection cycle.
If you want to keep your numbers down then the medical advice to use drugs or insulin (while the steroids are ongoing) is both inevitable and likely necessary in order to control your blood glucose. Some people experience a return to normal blood glucose once the steroid course is over, but it doesn't look likely that your steroid injections and allergies will disappear like that, so a long term plan sounds like it is pretty necessary.
I really do wish you well with this. Very difficult situation.