I have a friend who has set up multiple flat-bar bikes that were designed with drop bars in mind. He has had his pick of 130-150mm stems that no one else wants to make this work, because anything shorter makes the bike tiny and awkward like
c_m_shooter says. it took him years to come around, but he's finally started riding a proper drop bar setup on a frame that was designed with a drop bar in mind. he seems to still make them fit similar to a mountain bike, but it works for him.
There are quite a few drop-bar mountain bikes or mtb-ish bikes on the market like Black Mountain Cycles, Otso, Crust, etc.
Moisture will need to bump his budget if he wants something truly purpose built for that kind of thing.