Yes, I know other versions of making rice with mushrooms that do that - here adding the mushrooms earlier helps add flavor to the rice. If you saute some mushrooms to add at the end, I would probably go for some crimini rather than shitake but that's more of an aside. As for sake/mirin - I think you could do either, mirin is generally a little sweeter so just keep that in mind if you add more mirin. Another option, if you had it, would be to use sherry rather than the mirin, or even dry wine. I think mushroom broth would not add much since that's in effect what you have from the mushrooms being in there, but certainly wouldn't be bad either.

I don't have sake but I do have mirin, would I put more mirin or just leave the sake out completely? Would a splash of vegetable or mushroom broth be good in place of the sake just to add some flavor?

Wild mushrooms sound wonderful in there, as does the rayu chili oil on top. Glad to hear you have been enjoying!

I agree, they often seem to be that bit under-used. This is such a lovely way to enjoy their flavors, and indeed the couple of additions really bring out the umami flavor. Enjoy!

Thanks - they are so simple but also so flavorful!