Thanks for your replies so far. To answer one question - yes I shoot in raw and process in Lightroom. My ideal solution would be a 24-105 mkii with equivalent sharpness to the other recent releases from Canon, but it doesn't seem likely this is going to be released. Purchasing the 6D with 24-105 plus the 16-35 f4 will cost in the vicinity of $4000 aus, which I am prepared to pay to get the quality if the 24-105 on FF will deliver significantly sharper images than the 18-135 STM on crop. However if this isn't the case then I can probably do a 7d mkii body and keep the rest of my kit for around $1500 aus (guessing at the price here).
I accept the 24-104 won't be in quite the same league as the other 2 L zooms, but will it be significantly better than what I have now?
Well, sometimes my 500D with 18-55IS produced perfectly nice and sharp images and 7D wasn't that much different. Now 6D is just delivering it most of the time (instead of sometimes) even with non-L lenses. 40mm pancake is my walkabout.
There are no L EF-S lenses or even good+fast+cheap(affordable) primes (my weapon of choice) and I wouldn't be surprised if 7D'II cost more than 6D, so FF is a win-win for me.