A cast iron flange was typically attached to the cast iron pipe using oakum and molten lead, then caulking the joint (expanding the lead by pounding it with a caulking iron, not applying some goo). Find an old plumber who still remembers how to do this and have him reset your flange so that it's level. There's typically plenty of play between the flange and pipe to allow adjustment. It's not hard as a diy project, I've done a few. A blunted cold chisel will work as a caulking iron.

If it's some more modern system that doesn't provide any room for play to adjust for inconsistencies in leveling I imagine there might be a more spongey gasket that would work if the gap isn't too big. Can you use a wax toilet ring with this system? That's typically how small gaps in a toilet install are managed. I'd ask the manufacturer what their recommended solution is.

Thank you Rwiegand.

Howerver the building doesn't recommend to disconnect the cast iron flange from the cast iron pipe since it can break the pipe. It led in the past to leaks in below floors.

the issue with the toto is the little room under the adapter.