Mirrorless cameras were actually the enabling technology for me to get into astrophotography (10 years ago, with Sony's first generation of APS-C mirrorless). The biggest advantage over SLR is the much shorter flange height, so you don't need a scope with as much back focus. The lower weight was also a plus (I started on a Newtonian, so both the back focus and weight were key concerns).
Regarding your specific concern of focusing, I found the electronic viewfinder equal or better to optical. Because an SLR's viewfinder has a diffusing screen in it to simulate the focal plane, you're only getting a fraction of the light from the scope to your eye that you would with an eyepiece. There's a decent advantage in the gain and edge enhancement that the electronic finder provides.
Where I've gone in the past couple of years, though, is to ZWO's line of astro-specific cameras. They're similar sensors to what you would get on a normal photography camera (no low-pass or Bayer filters), but packaged up in a form factor and with electronic interfaces that are optimized for astrophotography.