This is not going to be an answer you have been hoping for. Sorry, but that's the way it is. What you have is an 8-3/4" rough-in, assuming a 1/2" drywall over the studs. That won't work on any toilet. Building a platform is definitely the sign of a hack job. It will be unsightly, and totally mickey mouse. My best advise is to break out the concrete to access the pipes and do it right. I would suggest a plumber to do the pipe work, but the neatest way to handle the concrete is to hire a concrete cutting company. You can do it you self, but it is a fair amount of work and can be messy. Certainly the pros charge for what they do, but it is done quickly, they clean up the mess, and it's done right. I would urge you not to go the platform route.