Hi! That's a big complication with this pairing, and I'm glad it worked for you.
I have only one other Harry/Sirius (actually, Harry/Sirius/Remus), and while it is longer, it's also quite different because it's a time-travel/universe-travel story, so Harry is a bit older than Sirius. There's still a trace of guilt, but it's not nearly as much of a focus. That's Making It Right on my Harry/Any Marauder comm, Up to No Good.
Making It Right is the sequel to a much shorter story, Worlds Together. Read the content warnings if you need to for some reason, but I recommend it without. (I will say that it has no character death or non-con.)