Aggressive Tendencies, pls help

I have an almost 4 yr old mutt (Staffordshire bull terrier 65%, Australian Shepherd 20%, white Swiss Shepherd 9%, australian cattle dog 6% mix) who’s always been cautious of strangers and other dogs, but last year he went through a trauma with me (my ex attacked me with him present, he has been more aggressive since then). we’ve been in a stable place where he has his own space for about a month now and he was doing better but our neighbors are outside often and leave their dog tied up outside (he can see him through the window) and he has been getting food aggression with our other dog (18 yr old chihuahua who he normally LOVES and has had no problem sharing with in the past) and he has just recently snapped at my roommate. i’m at a loss of what to do. i play with him when we go outside and i play with him inside often so he gets plenty of stimulation (he also has an arsenal of toys). He’s a very skittish dog and i’m not sure if that’s contributing to the aggression or not. i have also been working on getting him muzzle trained (it takes time, especially since he’s so skittish and cautious of the unknown). Any advice would help, i’m at a loss and im afraid he might snap and hurt the older dog or my roommate.