We didn't have any trouble at all with the spouse visa. One thing I used was the round robin Christmas letters I write every year - gave them about 5 or 6 of them printed out - they had photos, family news in etc and you'd hardly go to that bother if it wasn't real. Mind you, I don't think you'd have 3 children just for a visa either! Maybe 2, but not 3! :)
We each wrote our statement, our story of how we met etc, we also supplied a couple copy letters of our correspondence before DH moved to England on his working holiday visa etc.
I think if the docs are well organised, well presented, and as thorough as you can get between yourselves (copy anniversary cards etc too, I think we did a few of those) they are just going to be pleased you have done a good job of the application and it will be easy for them to satisfy themselves its all in order. Ours came back by return as soon as they'd got my medical stuff.
I really don't think you need a migration expert for a spouse visa unless you feel your circs are really complicated. Just got to be methodical and make sure you haven't forgotten anything. We applied only for me, and did that after we had got the boys their dual citizenship, so I was the only one without an Aussie passport.
I am just a bit annoyed that we hadn't by that point done DH's British citizenship because we will have to do the reverse procedure if we ever go back to the UK. We've done that once before when we got married (went to Canberra to get his spouse clearance) and that was pretty easy too. Actually that was a walk in the park we hardly had to do anything much at all for that one.
If you have children you can apply for permanent residence without any wait at all, and can get centrelink and all that without a residence wait, so it was very easy to get in the system here with that.