I am running Volumio since many iterations with an original Raspberry Pi 7" touchscreen and a HifiBerry Dac+ which also uses dtoverlay=hifiberry-dacplus without issues (I know that’s not really of help 
).
So my first thought would be about the connections of the display:
When you had the display connected did you use the dedictated cables for SDA and SCL? If so, try without them. They are only required for old Model A/B boards where the DSI connector does not have I2C connections. With your Pi 3B SDA and SCL signals are transmitted via the DSI ribbon cable.