If you have the 50 f/1.8 and it's still working, and that's all you have, then don't get 50 f/1.2 as your next lens... If you need to alter bokeh on your f/1.8 you can probably smooth it out a bit it post using a feathered selection and a touch of Gaussian blur. That said, my L lenses all work great on my crop body, you just end up cropping out large areas of what the glass can do and in most cases this isn't ideal.
I'd agree with the advice of switching to a full frame first if your intent is to spend thousands on a collection of super high-end primes. For a long while I thought the versatility of Crop + FF sounded great on paper as your lenses do different things on both, so long as your Crop bodies resolution is equal or greater than your FF's res... I don't think I'll ever buy a crop body again, they are best suited for Telephoto and Macro and even still I suspect that the same images taken on a full frame will have much more character. With a 5DIII, I suspect I would have no personal qualms about heavily cropping any image to sorta/somewhat match a crop and still have a fine, printable image.
I have an old used 5D classic and a 7D. The 5D I have can't do video, while the 7D can, and the 7D's resolution is greater than the 5Dc so there is no realistic way to crop a 5Dc image anything close to a straight 7D shot and still have the same detail/resolution in place. It gives me a backup having both but to be honest I wouldn't re-buy the crop if I could do it over, I would however get the 5Dc again in a heartbeat but the price of a used 5Dc was not reasonable back when I bought the 7D and I was at the time more concerned with video until I fell in love with stills. Unless you are doing a lot of f/8-f/16+ shots with a large DOF you are losing a ton of built in character on a crop body, and to me it's an utter waste and a big dent in your photography.
In a year or two if work treats me decently I'll probably upgrade to a 5DIII, the camera hit about 99.9% of everything I wanted it to be, and if I could afford it (fat chance), I'd seriously consider switching to Lieca after thorough test driving, it appears to suit my style better for photo only purposes, I've looked into just about every other system in the same range, and nothing else does it for me.