What kind of 600-700 dollar lens do you need for the D600? The primes are cheap, and will really show what full frame can do.
And honestly 4 lenses arent that much of a reason to keep yourself from switch brands. If you need FF, pentax doesnt have it period. I kept my K10D just to run the DA 40, because no other manufacturer offers anything quite like it.
I was in a similar, but not exactly the same position as you. I want to upgrade but I know I need the DOF control of FF, so switching to nikon was just something that needs to be done. You sound like you still have options. In the past I did entertain the idea of buying a 5D + 50mm alongside the pentax stuff, but I realized the AF just wouldnt be adequate for my use (yours might differ), and juggling two brands in a wedding would be a huge liability.
Just as a food for thought, I have basically replaced 35/2 and 50mm/1.4 with 28-75/2.8 on FF (not the same size I know, but matters little in a wedding). I got the 85/1.8 for the bokeh stuff, and the extra DOF control really helps.
Both 5D and 5D mkii have good rendering, I dont think it's just placebo. IMO they are indeed capable of higher highs than both K-x and K20D. However I don't think K-3 will lose anything to the 5D / 5Dii other than DOF . In fact I think the K-3 is more than capable of besting them. K-3 rendering with good glass is wonderful as well.
IMO rather than asking, what should I buy next, you should ask yourself, what is limiting you and what do you want to explore? If I'm in your shoes honeslty I'd grab some new lenses that you dont have yet (10-20mm, 10-17mm FE, a 35mm prime, any of the DA / FA limiteds). Keep some of the money in savings until you know what is limiting you.
You also talked about joy of shooting, Throughout all my time doing photography I actually enjoyed myself most using an ME Super + M40/2.8 and Takumar 50mm.... so perhaps pick up some film?