Over the past few days, I've been working on a revised PCB. It is now in production, so given international shipping will continue in the following weeks I'll soon be able to test it!
The following improvements over the original version where made:

• Switched to the ESP32-Wrover Module, which features 8MB PSRAM. This allows to hold full frame buffers in memory.
• A footprint for an optional connector for the ED060SC4 6" E-Paper display was added. This should allow for driving either a 9.7" or 6" display with the same board!
• As some ESP pins are now needed for the PSRAM module, some configuration pins where moved to a shift register.
• A temperature sensor was added to be able to (in theory) adjust the driving waveform to the environmental temperature.
• Smaller PCB outline of ~100mm x 32mm.
• The board can be powered directly through 3.3V (without USB) for lower power consumption.
• Instead of I2C, a full SPI port is broken out.

Meanwhile, there are still some things to improve software-side: Currently, the refresh time of about 1100ms is mainly bounded by the ESPs flash bandwidth. With better data encoding / compression, it could be lowered even further.