I have been shooting mirrorless since 2013 and I can say that things have improved a great deal in that time, especially when it comes to the EVF. Comparing my Panasonic GH3 viewfinder to the G9 is like night and day. So much more realistic. That’s going to be the biggest change for you from a DSLR - to look at a screen rather than through the lens. Because of that, be prepared to go through batteries more as well.
Dennis brings up the point I was going to lead with - weight and size. The only way you’ll save on that is to go to m4/3. The bigger the sensor, the bigger the glass. My 70-200 f/2.8 lens literally fits in my hand and you can still see the tips of my fingers.
The bodies of many mirrorless cameras are going to be of an equivalent size and weight to a DSLR and for good reason - human hands haven’t shrunk. Ergonomics are important and that’s why I went with Panasonic rather than Olympus back in 2013 despite being an Oly shooter since 1985. I couldn’t handle the camera efficiently. It was too small, cramped and I hated the rear screen. Olympus has since abandoned the effort to make everything retro, and their ergonomics have improved. Their glass is as terrific as always, but I chose to go with all Panasonic lenses in my kit. Micro four-thirds mount crosses three manufacturers - Panny, Oly & Leica so the choice is very wide. Marry your lenses and date your camera.
The flagship models in either Panasonic or Olympus should give you a feature-rich camera with IQ that is a match for just about any larger sensor rig. That’s what keeps me shooting this format - I don’t have lenses the size of horses legs and I have outstanding IQ and features. The lenses in my kit are high-grade and super portable. The other day I had two lenses and the camera in a hip holster. Focal lengths from 24mm to 200mm at f/2.8 through the entire range. Can’t beat it with a stick. If all my gear fell in the river, I’d probably buy the same thing again. The G9 is a joy to work with and I love it to bits. Panasonic has just come out with a new camera in a different model series, and I expect that the G9 will get a replacement, too.
Oh and I promise I’ll shut up, but one more thing. Panasonic is great about releasing firmware with huge advances in technology. They developed some focusing improvements for their full frame line and some of that came over to the m4/3 line as part of a firmware upgrade. And they don’t charge for it - it’s free. They do it for lenses as well so I have to say that it’s a great way to extend the life of the entire kit.
Hops off soap box.