Personally, I think most DACs probably do sound the same, but there are definitely some that sound different on purpose to differentiate themselves. There's a thread over on AVS using Foobar ABX to test the transparency of the $8 DAC inside the Apple Lightning cable dongle, and so far nobody has been able to pass the test(meaning it's audibly perfect). If Apple can produce an audibly perfect DAC for $8, then I assume most other manufacturers selling $100 or even $1,000+ DACs can as well. If hearing the music exactly how it was intended(transparency) is your goal, then one need not spend more than $8. Hard to believe that $8 buys perfection(at least until we start getting cyborg ears ala Cyberpunk 2077), but it does seem to be the case.
I may eventually try an external DAC, but it would probably be something like the Chord Hugo 2 or something else like it that has multiple presets for different colorations, like "Warm", "Detailed", "Neutral", ect; kinda like a form of tone controls. Plus, it looks cool. There's also the issue of different DACs supporting different formats, features, ect. which is another good reason to choose one DAC over another.