+1 for Carbonda 696 or 707.
Although supposed to be Gravel frames, I've seen some flatbar builds. The 707 is rather long frame, so this should work quite nice when you combine it with a longer stem, e.g. 100mm or something in that range.
Or you could go for a hardtail mtb frame...i think Carbonda also sells single forks, but don't know if they fit to the mtb frame.
I ride a 505 as Gravel and I hit so many deep and painful potholes on the streets, quite some impact for the fork.
I had to put the fork out several times, and each time i checked the fork.. it's absolutely fine, nothing visible. That gives me a good feeling about the strength of the frame.