You have many options for DAP, unfortunately they are often android based. In some rare cases, they are either pure custom OS, or stripped down android device. I have a Cayin N3U, I hesitated a lot with some Hobby device as it's a small brick and Android, while the android interface is still too present for my taste, and a bit too large, I have no regret, I really wanted something dedicated, no Bluetooth, tube, portable, no streaming, no extra app.

There was a merge into Rockbox [0] last month with someone making their own hardware [1] primarily to run rockbox.

[0] https://gerrit.rockbox.org/r/c/rockbox/+/6510 [1] https://github.com/amachronic/echoplayer