As a hater of hidden UI like Apple has been obsessed with for several years (force touch, most of the touchbar, edge swiping (as convenient as it can be sometimes), etc): there is a way, but why would you ever try it:

Touch-and-drag on the volume buttons to immediately drop into a volume slider, relative to where your finger started.

It's a pretty nice interaction, honestly - better in most ways than the buttons alone I think, in part because you can be both more precise and make large changes faster. It's one of the VERY few places where I think the touchbar does something positive[1]. But the vast majority of people who would like to use it have no idea it exists.

[1]: ... except when the touchbar freezes while doing this, and continually sets "max volume! MAX VOLUME! MAAAX VOOOLLLUUUUUUUMMEEE!!!!" several times per second until you do a hard shutdown to stop it from blaring [whatever you were listening to] at you because it's also stealing focus from whatever's on screen so you can't even click the "yes please shut down" button. Then I'm somewhat less fond of it.

Yes, I totally agree. I much prefer touchbar volume slider to physical keys, because you get near-instant granular level selection over large range as opposed to multiple key presses required with courser increments to move to a certain level.

I probably use this feature 10-20 times a day, and it would be a noticeable irritation for me if it was key based. I am disappointed they're dropping the touchbar in upcoming M1x/M2 macbooks, I would definitely have preferred a touchbar. Apple, are you seeing this?