R2R is short for Resistor to Resistor. Sometimes called a Ladder DAC. It's an old decoding method from the 80's involving a resistor for each bit. Modern DACs have one resistor and use complex math to break down and reconstruct your 16 or 24 bit files into a one bit data stream. This is called "sigma delta" decoding. BTW, DSD is a native one bit format but never really caught on, so today we still do this breaking down and reconstructing thing.
In short, it is much easier and cheaper to design a good sigma delta dac so you don't see too much R2R these days. It's sorta like tubes; strictly for the enthusiasts who like the sound or appreciate the tech, even if you pay more for (usually) worse technical performance.
The Topping E30 looks like a fantastic DAC. Personally though I've experienced really bad issues with Topping products arriving DOA (two DX7 Pros in a row!) so have sworn off the brand. Hopefully these issues have been ironed out, but one youtube reviewer has already discovered that DSD64 decoding is busted on the E30.