590 ETT and 400 reach is about what I would expect for a bike that would fit you
with a drop bar. if you put a flat bar on a bike with dimensions like that, you're either going to end up with a cramped, upright position, or you'll need to put a longer stem on it to compensate.you can do whatever you want, but there are good reasons why no one makes a bike that short with a flat bar in mind.
All City is making the
Super Professional with dimensions like that in the 61cm size and building it with a flat bar. it looks like they are using a mtb-style wide handlebar and a pretty long stem to make that work. it might not be the bike for you, but the geometry and build could be a good starting point for inspiration.
you have to ask yourself: do you want something that fits and handles like a road bike with a flat bar, or do you want something that fits and handles like a mountain bike? there's a difference.