With some basic SERDES knowledge I wonder what could be done in 2022, now that we have cheap FPGAs. Ronja is limited in that it piggybacks on 10BASE-T Ethernet, which in turn makes in simpler. It wouldn't be easy to analogously piggyback on 100BASE-T due to PAM-3 transmission. An FPGA would allow for a bespoke PHY/MAC to be implemented in lieu of Ethernet.

There are many techniques for pushing bits into a wire faster, like CTLE, DFE, pre-emphasis, to name a few. Probably the best starting point would be Behzad Razavi's "Design of Integrated Circuits For Optical Communications".

For starters, the LED driver could use current switching with some idle current (pre-bias) in "0" state to achieve more bandwidth.