The FiiO M3K FLAC-compatible player looks promising, https://accessibleaudio.co/2018/12/27/fiio-m3k-review-a-pock.... There's a partial native port of Rockbox firmware for the M3K, which improves on the stock playback UX, but is missing other functionality.
Firmware: https://www.rockbox.org/wiki/FiioM3K & https://fluxcoil.net/wiki/hardwarerelated/mp3player/fiio_m3k
M3K native port source: https://github.com/Rockbox/rockbox/commits?author=amachronic