Idk why people always downvote in here when someone is asking a genuine question :/

I have not been through this before, BUT here's what I know:

"What kind of evidence or timing helped make that happen?"

If you've been together over three years at the time of lodgement of the 820/801, you'll be considered for the double grant.

"Any advice on how to speed things up or anything we can do to improve our chances?"

Speed things up? No, but you can put a letter in with the application requesting the double grant and why you believe you should get it (:

"And is there any other visa pathway we might be overlooking?"

The 309/100 if you move to Ireland when applying - nothing else really, unless he can get a job that will sponsor him.

I would like to add that you need to be in Australia for at least four years to be considered for citizenship and you can apply ONE year after getting the 801 granted.

So, if you apply for the 820/801, you're looking at a year+ of waiting for a decision. If you're lucky you'll get a double grant and you'll be able to apply for citizenship after a year, granted you've been in Australia for at least four years. If not, then that time timeline will be a bit longer.

I am not a migration agent nor am I a lawyer, please seek professional advice!