I've recently built a big deck.....
11m L x 4.5mW.....
I made a box steel frame with steel legs, in my case the legs are on rock but could be heavy paving slabs....
where there are descrepancies in hight I have used 20mm threaded rod to gain correct levels......
I then used 150x50mm joists.....
this will outlast me.....
if I were in the UK I'd have had it galvanised....
but guess this is OK if u can work steel...
and lastly the decking is Swedish treated pine (virt no knots) 4.5mL x 120Wx20mm .....
only did 7 running meters and that was almost €800's.....