Not open source unfortunately. People have started reverse engineering the firmware update files though, so you can already flash a slightly modified firmware at least.
I don’t own any bluetooth headphones, so I haven’t tried it. From what I’ve read, it’s fairly basic support and not really the focus of the device.
My phone is a distraction machine. Having dedicated devices allows me to focus better.
It’s smaller than a phone.
It has a long battery life.
It’s a proper high-res audio device. Much better audio quality than what you can get with bluetooth, a built-in headphone jack or most dongles. Even has a balanced output.
And finally, it’s just more fun.
I also have one of those. Really nice, premium-feeling hardware and great sound quality, as expected from Fiio. They really should have added some additional buttons though. Forward/back, menu navigation and volume control all using the same 2 buttons is quite annoying. The software is also rather clunky and basic, comparable to the average mid-2000s “nugget” - I guess that might make it even more nostalgic for some. In any case, it’s cool having my own dedicated device for music I own again. Right now, it’s a solid 7/10 device for me, but could easily become an 8 or 9 with better software.
I do use the headphone jack. I was referring to a phone’s built-in headphone jack in the above comment, as that was the point of comparison.