I’ll echo what Joe says, when I had my FT-817 the SSB Mic gain setting was between 50 and 55 - 90 is too much. I presume you are using the standard MH31 microphone.
On my side, In my activation yesterday I proved that the RFI problem is nothing to do with the DYC-817 as I was getting the problem without the compressor connected and the problem (raspy audio) went away when I turned the 50-70w amplifier off. I have now added clip on ferrites and about to do some “cable surgery” to hopefully fix the pick-up. Once I have done that, I can go back to testing the compressor!
I am surprised that you have worse reports with the compressor turned on than off Guru. Try turning down the internal Mic gain inside the FT-817 and make sure, first, that all is still OK without the compressor in circuit. What I am wondering is if the internal gain is so high, perhaps the microphone insert in the MH-31 isn’t working as well as it should be and might need replacing?
Is the switch on the back of the MH31 on “1” or “2”? One setting is supposed to give a more “rounded” - aka ragchewing audio - I think that’s “2” if I remember correctly. For our use it should be on the communications quality setting.