Why don't you consider some of the di2 mountain bike stuff instead of the road bike stuff. If you already have Ultegra or DuraAce STI's, I don't know what the implications are for that. Maybe someone else does.
But if you find a 36 tooth Shimano compatible cassette 11 speed cassette, I suppose it can't hurt to try it. Might need a hanger extender. Might want to consider something with less difference on the front than the max of 16. If yours is a 53/39 front, then you might be good with that.
Last edited by Iride01; 06-01-21 at 05:29 PM.