Advise on road/commuter bike (flat bar roadie?)

So I want to dip my toes in the water of road biking and commuting, my mountain bikes are just silly on the road, not the right tool for the job, but I don't rest want to go full spandex boy either. 

I want to be able to put some miles on with comfort a priority over speed, and maybe also have a bike I can use on gravel paths with my kids. 

I was thinking flat bars - or does that just limit me to mostly junky bikes? 

Kona unit x, or maybe a few?

Some manner of surly or all-city? 

Would prefer to keep it under 2k but would buy used to improve buying power. 

Am I better off with a drop bar bike with more MTB-rider-friendly geometry? 

Norco Search xr steel? Giant anyroad? Bombtrack hook ext or santa Cruz stigmata? (getting expensive there).

I guess everything is relative but what's our thoughts on a good compromise between not being too flimsy/uncomfortable/racy but also not being a slug on the road and basically a mountain bike? 

Any thoughts much appreciated!

I‘m on last years Bombtrack EXT-C, have you looked at the 2019 steel Audax? That‘s what i would getting now, considering how i use the bike most of the time.

Mountain bars are uncomfortable for putting on miles on the road. Drop bars take some getting used to but are a far superior option for roads (and gravel). The ergonomics on your hands and wrists is one benefit. Being able to change your position is another and finally, aerodynamics if you are trying to go faster or hit a strong headwind.

In terms of what to buy, I have to say that the benefits of a carbon road/gravel bike are huge. I'd buy used to crack into that market over a steel or aluminum new bike. It is hard to know what to recommend without knowing what you ride on mountain. What is the height differential between your saddle and bar on your MTB?