I make a Teensy 4.1 based sound effects processor and have my guitar preamp very close to the Teensy and audio processor.
I can hear some digital noise on the guitar input and want to minimize it.

I try to separate all analog stuff from the digital and put all the good capacitors, but wonder what else I can do.
One thing could be to fill all empty space on the board with grounded copper, like some people do. Others say it acts as a capacitor and makes things worse.

Please share your experience.
In general, what do you do to lower digital noise in analog inputs?
Thanks.