I thought that the two focus knobs would be horizontal and vertical focus, but they weren’t. Not sure how they work but there are only four options to try out (increase or decrease knob 1 or 2) so didn’t take long to do.
The 21” version was supposed to be solid but I have never seen anyone with one or seen it for sale. The price tag was probably too steep compared to the faulty 17” one. Sad to hear that the graphite model was just as bad though.
Sorry, my wording was unclear. The USB RTL-SDR was for receiving, and I did the capture and analysis on my desktop. The Raspberry Pi was for transmitting using the GPIO pins.
I went into the weeds of reverse engineering my Minka Aire ceiling fans with a USB RTL-SDR. In my case the manufacturer's remotes worked fine, but I wanted to integrate it in with Home Assistant. If you're interested, you can see what I did: https://www.riveducha.com/decode-wireless-signal-with-usb-tv...
I used a USB RTL-SDR ($15) and a Raspberry Pi to receive and transmit, but I think the Flipper Zero can do it all-in-one.