Plumbing – How is this toilet flange attached

There isn't much allowance for raised flanges in most US toilets I've seen. The critical feature is how much higher the under toilet "ceiling" is than the bottom perimeter base that sits on the finished floor. Since this dimension is intended to work with a standard wax ring, there's little chance a 3/4" protrusion will work. Not only do you need physical clearance between the flange and the "ceiling", there also must be room for the wax sealing ring.

The other potential conflict is the outlet horn and how it fits into the small bevel of the flange fitting. Again, there needs to be room for the wax to fill this gap. You do not want any direct physical contact between the flange and horn.

Simply supporting the flange is not enough. The perimeter base must be fully supported. If you try to install the toilet as things are, the toilet will be balanced on the wax ring. It will wobble every time it is sat upon, eventually forcing all the wax out until the "ceiling" is bearing directly in the flange. While the wobble will diminish at this point, it will still wobble. The seal would obviously be compromised.

Whatever you do, the perimeter base must be solidly supported. The obvious solution is lower the flange, though very difficult in your case. Another solution would be to raise the floor, which I imagine would be undesirable. The only other possible solution would be to raise the support of only the toilet base, such as setting it on a grout bed. Depending on the nature of your flooring and the actual final thickness of the grout, this may not be too bad a solution. You would need to spend enough time finishing the exposed grout around the toilet base so that it is smooth and easy to clean.

Check out some of the fittings and repair parts from Oatey. Not sure what sizes of pipe you have. But if you have a 4 inch pipe, 43539 is supposed to slide inside of it. It has a rubber piece that expands using 3 socket cap screws.

Link to Oatey site product page for the fit inside, expanding rubber flange

I have been repairing my own toilet bend, which is a lead piece coming from a cast iron drain stack. I would have used that part above, but I think I'm going to go with a 3x4 repair flange instead because this non expanding flange was shorter and fit better. The "3x4" part means it will fit over a 3" pipe or fit inside a 4" pipe.

I hope this helps, every toilet flange problem is different. I had to buy many of the repair parts and test fit them before I found the right piece.