Hardware volume control on a DAC with Volumio defines the way the volume control is handled by the audio player, hardware means minimal (no) loss in audio quality.
What you describe is volume control knob on the DAC it self.
The only DAC’s i can imagine that have this are USB connected and costs are higher than the various i2s PI DAC’s
for example this one: thomann.de/nl/fostex_pc_100_ … tAodQWIA0g
With the Raspberry Pi (and many other platforms) it is possible to use the GPIO. The DAC’s don’t use all the available pins, this should be documented somewhere or ask the supplier.
This can be done using a “Rotary Encoder” like adafruit.com/products/377 combining this with a script that increases / decreases volume according to the movement. This rotary encoder uses 2 GPIO’s and ground.
These a guide on reading a rotary encoder with the R-Pi here: theatticlight.net/posts/Reading- … pberry-Pi/
I’ve looked into doing this before, but didn’t continue since two simple buttons ( + and - ) were easier for me and i already have those buttons