AusBonsai

I've been using Iron Bark for benches.. both 2" and 3" thick. Despite them being a more expensive timber, they are solid and do not warp. Even Red Gum warps but Iron Bark has been really good. I was also a bit concerned regarding the chemicals they use in treated pine.

It looks much neater, and you'll get better coverage of sun on your plants with the tiers. Since it's temporary, you're at least giving the plants the best options you can. Well done!

My own situation isn't much different, in that I have piled all my best trees into the area that gets sun right through Winter. In Summer though, the plants can't stay in the same position. The sun moves so much that only a fraction of the area gets Summer sun, plus the whole area is way too hot for the majority.

I have a seasonal dance to shift plants from one side of my yard to the other....and I suspect you will too. Personally, I think your plants will be happy getting good Autumn, Winter and Spring light in that aspect, but you might have to find some shelter or shade cloth for them in Summer.