I would try a re-design - which could eliminate a lot of (expensive) ironmongery:
One option would be to arrage for the back beam to be a few inches higher than the front one, with through mortices (roughly square I guess) cut into it. You then cut tenons in the ends of each of the 2m 'rafter' pieces. If you wanted to get fancy-pantsy/oriental-stylee you could make wedged tenons.
The advantages, besides eliminating ironmongery, would be no holes in the top surface of your timbers to collect rainwater.
Don't forget that the bases of your posts supporting the 'back row' will be trying to rise up and will need to be very securely attached to your base plates.
e.g. a bit like this:
View attachment 118691
that's a really nice idea! similar to what I modelled in post #18 based on a suggestion on here - I wasn't too keen on that design in the end because the height differential between the front and rear support beams didn't quite look right to me. I might have a go at modelling your idea up, I quite like the fancy wedged tenon version too
the only issue is the spars sitting above the posts - I would need to use screws for those