Palette wood won’t last very long because it’s not framing lumber. If you don’t mind repairing it in 2-3 years (depending on your climate) then proceed
after you pick up from the City Building Dept a copy of their Prototypical Deck Framing & Foundation requirements.
It will tell you what kind of spans, blocking & fasteners they mandate.
If your deck isn’t sitting right on the ground, it will have to have guard rails, if it is 20” * above the ground.
Labor is labor , why waste your time with inferior wood that won't hold up to the weather and could be contaminated? That's why they throw out pallets, many times what they carry gets spilled
There are a bunch of ideas over here: https://www.hometalk.com/search/posts?filter=pallet%20desk
Here is an idea from a blogger https://2beesinapod.com/2016/10/31/build-deck-less-250/