I’m looking for an 800mm bar with lowest upsweep and back sweep.

The Thomson 800mm is the lowest I can find
6 degrees back sweep
4 degrees upsweep

Can anyone think of anything with less upsweep?

rotate the bar = zero upsweep. Do I win?

Broom handle.

As reggie said (or implied) the distinction between upsweep and backsweep is meaningless unless there is rise. The amount of rise you get from sweep depends on how you angle the bars and where the sweep starts from, but won’t be a lot.