Wow, Joe - I feel like I could have written almost EXACTLY the same post, save for a couple of minor details. In fact, I probably HAVE written posts almost identical to this one. I was heavily into photography for several years in adolescence and early adult-hood, but took a long break from most of it and only got BACK into it heavily about five years ago - my first immersion in digital photography. I too was strictly an m43 shooter, but then also got pretty heavily into the whole Fuji system after my experience with the X100. I think those are the only slightly different details. But then the RX1 came along and pulled me into full frame in exactly the same way it's done to you. When I first started shooting with a DSLR (about a year ago) I had the same reaction as you did - rather than feeling too big or too heavy, it just felt RIGHT to me. At least with reasonably small lenses - with the big guns, DSLRs can get out of hand pretty quickly. I don't shoot with any really big lenses - my largest, which I rarely use, is the 70-300, which is about the size and weight of the Oly 75-300 and Pany 100-300, albeit with half the reach. Today I took a 6 mile hilly walk with my Df, three Zeiss primes, and an old 75-150 f3.5 "E" lens that's quite small and incredibly versatile, and I was perfectly comfortable carrying that amount of gear. I'll almost never carry more and often carry less.
I look forward to your impressions and your photographs. Mine are that the IQ from the D610 is very slightly better than the RX1 in some ways - the sensor is the same but you can choose uncompressed raws which Sony doesn't let you do. But not quite as good in others - the way the lens and sensor interact on the RX1 is really something special and not something I think you can quite get with an ILC, although with the right glass it comes VERY close. I read nothing but great things about the Sigma 35 art lens - I suspect you'll really love that - although it's a bit of a beast!
Have fun,
-Ray