The correct thing to do is level the floor but if that's out of the question, it will be difficult to find a solution that will look nice.
If I were to try something, it might be epoxy mixed in with a thickening agent. Colloidal silica is white and you would end up with a matte white joint. I would apply tape to the side of the toilet and then use a plastic drywall trowel to contour the very thick (peanut butter consistency) epoxy under and around the bowl. You'd want to protect the floor as well. Make sure the epoxy does not bond to either the floor or the toilet so that the toilet can be removed if required.
And the only reason I'm even suggesting that is because I have both a gallon of epoxy and colloidal silica on hand. I use both as a fairing compound so the learning curve for me would be negligible. But it's probably not easy to work with your first time and it would be expensive to purchase the material.
An alternative would be to use shims and silicone. But for the shims, I'd use some thick plastic stock. Not sure what's available to you but UHMW square bars or something like a 3/8" cutting boards that you can get at Ikea would probably work. Cut the board to size and stack a few pieces together. And then get creative with a backer rod and silicone.
Are you sure you can't use the regular plastic shims you would use for toilets? They are about 1/4" thick so you would need to stack 7 or 8 of them to get the thickness you need. They have holes in them so you can screw the stack to the floor with a SS screw and then put the toilet on top. A few stacks of these at strategic locations would work. And then backer rod and silicone.
But fixing the floor will give the best results.