I think you are over thinking this. Why not just choose a mass that is well over anything expected and build it to that, eg. 1000kg. For a 1 metre span the additional structure is not going to be great. Call the additional steel corrosion allowance.
Yeah, tend to agree.
A few years back I built a series of timber bridges around my dam in Gladstone. The first one was with left over outdoor grade treated pine meant to be used in a deck. I built a 6m long box girder bridge, single span. The size of the box girder were 1m x 1m. Basically a copy of the Boyne River Railway Bridge style common in railways from 100 years ago, both timber and steel.
I used to drive my Ride-on mower - JD, 230kg + me + fully loaded trailer full of dirt another 200-300kg. The bridge didn't even creak. I did not engineer it.
I think the most important part s to spread the weight. Your axles will have around 150-200kg load max, just spread it across a few spans.