I don't know where you're located but all the deca I get is dosed at 250mg/ml. So I'll assume that's what you're talking about.
Whoever told you to take 500mg of deca a week for 5 weeks with no Test to make your shoulder stronger has no idea WTF they're talking about.

First of all, the first shot of deca is going to shut down your natural testosterone production. Second of all 5 weeks is about 7 too short for a deca cycle.
And it's not going to do anything to make your shoulder stronger. It might alleviate some joint soreness and cause you to overuse and further injure your shoulder.

So basically after five weeks you'll have low T, ED and your shoulder will likely be worse off than it was before you started. Does that sound like a good plan?

I'd say you should continue to do PT and get recommendations on what to do to naturally strengthen your shoulder and supporting muscle groups.
Go to the gym and work within your physical limitations. Put in the time and effort to make sure it heals correctly. Build it up slowly. The right way.