No need to go that hot. Ultrasupercritical coal fired steam plants run at 620C no where near hot enough to melt high tensile steel these are the top of the technology pile for steam technology. Just going to suoer critical is good enough when thr heat source is effectively unlimited like deep rock geothermal the extra 5% in turbine gross efficiency is not worth going from 500C to 620C most coal plants run in the 300 to 400C range you could just replace the coal boiler with two deep bore holes and a pump system with steam clean up if you direct cycle it or better yet run a indirect loop cycle with distilled water on thr power island side. You lose 20C in delta T but then you don’t put geofluids through your turbine.
https://www.power-technology.com/projects/yuhuancoal/
An effective use of geothermal wouldn’t even require 300 to 400C water...in a hybrid system powered by geothermal and conventional fuels. Using the geothermal source to preheat feedwater, then pumping that already hot water into a gas or coal fired boiler, could still greatly cut down on the amount of conventional energy needed to produce electricity. This would also increase the number of geologic regions where geothermal could be harvested.