Figured something out today that in hindsight seems rather obvious

Even with perfect measurements it's impossible to get to that line straight unless the frame is perfectly square. This not only completely solves that issue but also saves a ton of time not having to measure and cut them one by one.

Also the lumber I picked turned out to be 10 cm longer than the deck and I even picked the exact right amount. By the time I was done I only had a basket of <10 cm off-cuts.