yes he does deserve an Order of Merlin, First Class and I’m sure Harry made sure that happened
I’m 100% certain that the new Minister for Magic, Kingsley Shacklebolt, knew Sirius deserved it too, and at some point the conversation went like this:
Kingsley: … thought you might accept the Order of Merlin on Sirius’ behalf
Harry: I’m terribly sorry, Minister, but given how Sirius felt about the Ministry, I’m afraid I’m going to have to tell you to stick it up your arse.
Kingsley: *wiping a tear* It’s what he would’ve wanted.
This is everything