Really nice project Rolf!
Besides, just wanted note how elegant and intuitive the schematic drawing is. Beauty!
For the FX module, perhaps this could be an inspiration. Some time ago i started to work on an adaptation of the Jeannie Synth, with most components being SMT, AUX in (using codec instead of an DAC only), already built in FV-1 and other tweaks.
One of them was adding a 2nd module with Teensy4.0 used as a pure FX processor only, acting as "middleman" between the main T41 and the DAC.

The FX Teensy module would work as I2S slave, receiving all the clocks from the main T41 and being inserted in the I2S data line. Bypassing it, if the module in not used is easy by bridging the Send/Return I2S data pins. Parameter control would be done over I2C.
In the end i did not finish the project, it has some HW changes, which would require to mod the original Jeannie firmware. I wasn't sure if i will have enough time to do all the changes and maintain the alternative version. So it ended up being just a design excercise.