I second the Inteset recommendation. I've used it myself. While it's not RF, it does allow IR control from ANY universal IR remote, not just the included one. On Amazon, you can also just buy the dongle and splitter for $24 if you already have a universal remote. Just realize that, being IR, it does require line of site with the dongle and it can't do voice commands.
Another benefit of the Inteset remote is that it's JP1 programmable, meaning you can program it from a PC or Mac like a Harmony if you buy another $5 interface cable for it. In terms of functionality it actually runs circles around a similarly priced Harmony, giving you unlimited devices, nested macros, fast macros, 5 functions per button (primary, long press, double press, shifted and double shifted), pronto hex import, and more.
Having said that, I've gone the other direction and just use the Fire TV remote to control everything, at least in terms of power, input switching and volume control. It's possible if all your devices support HDMI CEC control.