This is a great question and I'm glad you're asking it. Once you get up to 90-100dB SNR I think most people would say you're well into transparent territory with decent headphones. 120dB is absolute state of the art (and the limit of must measurement gear!) but the difference is academic at that point.
Those mini dongles are pretty good, although they're not necessarily exactly world beaters. They're DACs + amplifiers and their amplifier sections don't exactly deliver a lot of current. But for e.g. desktop use you can get a really good DAC + headphone amp combo from Schiit, JDS, Topping, or others for $200ish.
Here's how cheap USB-C dongles from Apple and Google stack up with objective measurements: https://www.audiosciencereview.com/forum/index.php?threads/r...
Those mini dongles are pretty good, although they're not necessarily exactly world beaters. They're DACs + amplifiers and their amplifier sections don't exactly deliver a lot of current. But for e.g. desktop use you can get a really good DAC + headphone amp combo from Schiit, JDS, Topping, or others for $200ish.