@MichaIng Did some more testing, so far works perfectly.
As for how the protocol negotiates with clients, I can show you, but in typical Apple-style, there are almost no settings or options to choose - everything is automatic, which is good and bad I guess. The highest quality protocol is automatically chosen.
Laptop:
(shairplay-sync on dietpi test - YouTube)
Phone:
(dietpi: shairport-sync info - YouTube)
Multi-Device/Room streaming works also. Quality seems great, while there is few milliseconds lag, this can be corrected in the config file, and is not caused by anything DietPi.