macOS Catalina + Mac Mini: How to (reliably) use external speaker + control volume with media keys?

In Short:

I have not been able to find a reliable solution to use external speakers + media key volume control on macOS Catalina.

Is there any good / known solution for this?

Long Story:

I haven been using macOS 10.14 on my Mac Mini 2018 without any problems. An external display including speakers is connected via HDMI.

Choosing the display as output device in the system preferences "Sound" is now problem. Sound is than played using the display speakers. However, I was not able control the volume using the media keys on the keyboard which is quite annoying.

To solve this problem I have been using SoundFlower + SoundFlowerBed which worked fine.

Last week I have updated to macOS 10.15.3 Catalina. At first everything worked fine as well but soon I notices that every now and than sound stopped working. The only solution to get sound back was to restart the Mac.

Since SoundFlower is not developed anymore I found other questions / threads which recommend Blackhole as replacement. However, I was able to get sound back using Blackhole. Especially controlling volume using the media keys seems not to be included.

After some more search I found SoundControl which seems to work most of the time + allows controlling the volume using the media keys. However, media key controls also stops every now and then... It would be OK to pay the full version for $15 (however it is quite annoying that macOS does not provide a solution it self...) but of course only if this solution works all the time.

How to solve this?

I have had this problem before, especially since I use a Digital Audio Controller (DAC) connected to my Mac mini (2018), connected via USB.

To solve this, I used a piece of software called SoundSource, which supports "Super Volume Keys":

Enables keyboard volume control for audio devices that do not natively support it.

As my DAC is just a controller, not an amplifier, it does not have native volume controls, and yet I'm able to control Mac's output volume thanks to this little trick.

It's a paid software ($35), but has so much more additional functionality you may like, I think it's worth it.

Despite my best efforts, I have not been able to solve this problem without resorting to purchasing a third-party software, but maybe you can use Feedback Assistant to send feedback to Apple about it?