Thanks Michael. I can confirm that the Android 9 work done for build 223 now allows it to install on the fork for my Amazon Fire HD10+. 
It’s fully functional it seems, but I do note some really jarring ‘flashing’ in the ‘genres for you’ boxes on the home screen - see attached link to video for an example - when landing on the page or scrolling.
I only get this for the Genres for you section on the home screen, and e.g. not when tapping through to Genres.
(Currently I use Roon without a streaming service activated so the home screen has only a limited number of sections. Perhaps similar issues could occur with additional home screen content rendered when streaming)
Arc 1.0.45 build 100223 / Fire OS 7.3.2.9