@Lambeth I think you’re on the right path. You’ve already found the page that shows the pins that can be used safely with the PiSound. Any I2C LCD should work. I was going to include the link to the ones that I used to build my custom MIDI pedalboard but they are no longer available. I would also suggest Adafruit but at this time they don’t have any 20x4 units listed on their website. If you shop on Amazon what you’re going to find is you have to buy multiple LCDs to get what you want. But that’s okay too because if one dies you have a replacement. Here’s one that should work but whatever you do make sure the LCD already has the I2C module attached. I bought one without not knowing that and wasted some money.
I haven’t personally tapped into the available pins from my PiSound so I can’t really speak to how this should be done. Since the PiSound sits completely on the Pi I’m assuming you would have to solder a new header onto the top of the PiSound? Hopefully @Giedrius can comment on how that would be done.
Once you figured out how to tap into the pins this site can help you through the process of getting everything connected: How to Setup an I2C LCD on the Raspberry Pi
What I am curious is what information you want to display on the LCD. The reason I ask is you could also consider using a Pico with the 20x4 LCD. That’s what I did on my custom MIDI pedalboard: MIDIZilla for MODEP
Of course I had to write some custom code to run the pedalboard and output the relevant information to screen so if you’re not comfortable with coding that might not be a good alternative.