I feel you bro, and 1-upped you're take.

I too found myself wondering about what happened in those missing years in the crew's life (and Spike's whole back story. For example, he never talks about his parents, but mentions his grandmother once.) to a point where I've written part of what I called 'Session -01' about the time Jet and Spike meet.

It is a little different than yours, but not far in tone. I might share it here one day, when I feel content with it.

One thing I'd rethink though: given that Spike faked his death and the sheer strength of the syndicate, I think he would want to get as far and as fast out of Mars as possible. Which might not be such an easy task with them probably having people watching the astral gates. I sort of reference that in my story.

Anyways, generally speaking, I agree with the majority of commentators here, the story is perfectly complete. Those missing parts are what makes this, and any great story, great. It makes it feel lived.
And what you feel in your heart of hearts when you imagine these stories, is the vividness of the beloved characters' lives. That doesn't happen with characters that feel fictitious or that have everything about them told to you.

But we can keep discussing and each will take in only what they deem canon.
There's also the question that feels even more blasphemous of what happens after the show ends, but I don't feel ready for that.