How to repair toilet flange that is glued and recessed

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.

I may be seeing this wrong, and correct me if I am, but I believe the last elbow to the flange is a street elbow, and the flange fit OVER the small end of the street elbow. The fix may be a lot simpler and cheaper than you think. There are toilet flanges that glue into the interior diameter of the waste pipe. Since the last elbow is a street elbow, the inside diameter of it should be the same as the inside diameter of the straight run Sch40 pipe the flange I refer to fits in. Saw off the "bell" end that is left over of the old flange. That will get it down to the end of the street elbow it was glued over and fit the new flange in.The length of the stub may be a concern. Check this out....