The built-in DACs are better than most audiophile DACs; audiophile companies are largely scams and either way don't have the budget to actually do much R&D.
You may still need an amp for electrically incompatible (high impedance) headphones.
The Mac mini 3.5mm jack advertises "advanced support for high-impedance headphones" so you might not even need an amp unless you have some really crazy cans.
My M1 MacBook Pro (the first one in the new chassis) can easily drive my 250 Ohm DT 990 Pro to uncomfortable loudness so it should indeed be fine for 99% of headphones. I've been told it also drives 600 Ohm headphones just fine.
There are a lot of options out there now for USB PC speakers that have the DAC/amp built in, eg these are $50 and natively USB-C: https://www.amazon.ca/dp/B08F57GSJ7
But yeah, anyone "serious" would go discrete for all that stuff regardless. I guess this also lets Apple sidestep a bunch of fuss around non-stereo use-cases, for people who want quadraphonic or 5.1 at their workstation.
