I’m interested in buying a 2ft passive dac for a R4 in the future. Any recommendations for connecting to a cheap switch? Or anything I should watch out for?
I think in Serve the Home’s video they mention that there can be issues. I bought a switch similar to QNAP QSW-2104-2T-A
I have the QNAP QSW-2104-2S not cheap but works well (uses a Broadcom chip). Since I brought it Realtek has released its own chip and you can now find copy of that switch for about 30-35€ on Aliexpress (link) and they should works as well.
I’m using this https://www.aliexpress.com/item/1005005780965130.html with my R4 connecting it to my Zyxel xgs1930-28 and it works flawlessly.
I am using AOC cable https://www.aliexpress.com/item/1005004075266892.html without any trouble.
I also tested some SM and MM modules with LC cables, always connected to the https://www.aliexpress.com/item/1005005503868174.html directly, and everything worked as expected.
In fact, there is hw compatibility list with some sfp already tested, take a look and feel free to add what you tested: BPI‐Router‐Linux Wiki · frank-w/BPI-Router-Linux Wiki · GitHub 
XINCHIP XC-SFP10G-DAC0.5M cable works great for me. Is there a way to find out what chip the DAC is based on from the command line?
A DAC doesn’t contains any chip only wires and connectors in fact it’s in the name Direct Attach Cable.
That’s incorrect for cables longer than 7M. The signal decays enough to warrant chips to boost the signal, those cables are called active DAC instead of the cheaper passive DAC used for short distances.
For longer cables you have AOC not many peoples uses active DAC as they cost more and dont go that much farther (<= 15m) than a passive DAC.