When I purchased the R3, the current firmware version was 3.2. Like you, I also have no interest in Tidal/Qobuz and use this strictly for access to the SD card.
I've tried the 3.3 firmware and also downgraded to all available versions (in effort to find the best version)
I am a big fan of Snoopy112's firmware modifications. With his firmware, we are not limited to 20,000 songs. This was actually the deciding factor for me and the reason I made a purchase I couldn't find a "Snoopy 112" version for firmware 1.15, but noticed that he had a firmware version for 2.1
In addition to unlocking the 20.000 song limit, I also like the User interface layout better.
I'm currently using with no problems firmware version 2.1 (the Snoopy 112 version) .
The only lagging is a one second album-artwork delay (I have about 22,000 songs) This occurs with every firmware version that I've tried, but believe that 2.1 seems to handle this best.