Personally, I find it much better to use a tablet and mirror the screen to the TV, rather than using the app directly on the TV. That kind of works with FireTVs, you just need a Firecast app on the tablet, or use a Fire Tablet.
(Though Firecast in my experience has more lag and it’s less reliable than Chromecast. I don’t know why Amazon had to pick this fight and doesn’t simply support Chromecast on FireTV - you have to complain to Amazon)
AirReceiver is an app in the amazon app store, that allows the fire tv stick to be seen as an airplay receiver, or as a chromecast receiver. Though this does not work for me (chromecast that is.) Silk is the web browser built into the fire tv. I just use it for the roon display. I still use a tablet for roon remote.
I found installing sideloaded apks = sometimes they would say unable to install from unknmown sources, I kept trying and then they worked and installed. Once installed, most sideloaded apps Iincluding Roon) wont show an icon on the main Echo screen or in the new Fire TV section, you need to access the icon via the TAOTLIST app (as shown in the above video)