Could I improve further?
The RPi-DAC SingleStereo has some space for optimization:
Or you can use parts, mainly resistors and capacitors used in the signal path, with higher precision, e.g. 0.05% or 0.01%
All other parts, e.g. the LDOs or the entire DualMono, they are already optimized, to use the best parts available at the market.
Potentially, you will not realize an improvement on the audio quality listenting too. It might be better to check if you can upgrade the power amplifier, headphone etc.
Can I modify my setup later?
Sure, if you want to convert a RPi setup later into a SPDIF/TOSLINK/USB sond card, you can substitute the Raspberry Pi by the RPi-DAC-SPDIF module. You can modify setup anyh time later for other use cases.
The only limitation: if you use a RPi-DAC SingleStereo and want to change to DualMono - you have to change all modules: you need also the RPi-DAC-SPS module and the SingleStereo RPi-DAC cannot be reused (substituted by two DualMono modules).
Even you run without the SPS module - you can add and upgrade anytime with an SPS module (which give you better heat dissipation, higher power rail noise and hum rejection and additional option to power another board, e.g. the Raspberry Pi).
You can change a setup with RPi later with (instead of RPi which is substituted):
If you use with the RPi-DAC-SPS with transformer - the voltage, US (110V) or EUR (230V) can be changed via jumpers and does not need any changes of components.