I'm going to tell you what nobody else has bothered to tell you - the truth.
Something about what you are doing SUCKS.
Either your training sucks, your diet sucks, your sleep and rest sucks - or all of the above.
DS's test isn't underdosed, you just don't know what you're doing.
My guess, is that you don't eat enough. Steroids don't magicly build muscle. Well, they sort of do, but I'm not going to go there for this post.
My point is, is that you are not eating enough. 9 weeks, with 5 weeks so far on 500 sustanon 250 - you shoulda gained 7lbs of water alone probably from that - and that's just me pulling that outta my ass.
You need to increase your calories and you need to push your training hard.
Testosterone at supraphysiological ranges will build muscle excellently, improve your CNS efficiency - making you stronger, and improve your tolerance to volume, intensity, AND frequency of exercise.
I would NEVER recommend a natural guy train more than 2-3x a week, and never a bro split - but on steroids you can do that and make even better progress - because of the rapidity with which your muscles repair.
In general, as a guy gets stronger, it's his nervous system that cannot recover fast enough, which requires more time between workout sessions for the CNS to be able to fire off at full strength so that your muscles can display the strength you have gained through exercising the muscle.
EAT MORE FOOD.