I have Izotope 9 and 10 for MacOS and I found it's usage within FCP to be rather limited. What I do myself, I suspect is what you've been doing as well from the sounds of it which takes a little longer but the outcome is better.
I edit the video, export the audio, close FCP, and open iZotope 9 or 10 (depending on what I am doing, I've found depending on situation 9 can handle some things better than 10, and vice versa) and drag the wav file into iZotope and do my edits (dehum, denoise, whatever) and then once I've rendered it and it sounds the way I want, I export that as a mp3 file to my desktop. I then close off iZotope, open FCP and import that audio into my timeline.
One bug I have found, and I had this on my intel 19' Mac Pro as well as my current M1 Mac Studio ultra is that the sound or rather, coreaudio seems to get locked sometimes by one of the two apps so you will either not have sound at all, or the sound will sound muffled as if slowed down or a deeper pitch. Very strange but with FCP or/and iZotope closed, terminal command ( command sudo killall coreaudiod ) releases the audio so it's able to be used again without issues.
In iZotope there is a preference checkbox to release coreaudio when not in use but I'm not sure how well it actually works as the only fix I have found is that terminal command. Like you said, this is the longer way of doing things but I suspect iZotope is one of those classic Windows world apps that they say plays in the Mac world but it's nowhere near the same. Hope you find a faster way!
