Downloading 800 albums one by one. Wow, you have more patience than me!
I want to do exactly the same as you (although in my case to an iPhone) but for now I’m reluctantly going back to keeping a shadow copy of my Roon Core library files in an iTunes library and using iTunes to sync it all to my iPhone and using the Apple Music app for playing offline. I had really hoped that Roon ARC downloading would have given me a solution that didn’t involve iTunes but sadly it isn’t there yet.
By the way, I posted a feature request that would solve your issue by allowing you to do a select-all on the album view in Roon ANC and then use a drop down menu at the top to perform an action on all selected items, in our case that would be download to download every album selected all with one click but play, queue or any other action applicable to an album could be other options in the drop down menu. If you agree that such a multi-select including a select-all capability would be a useful feature to add to Roon ARC then please vote for my feature request here - Allow multi-select and select-all for albums, artists etc in Roon ARC library screens