It looks like the FiiO M5 is discontinued. At least, I wasn't able to find any store still selling one.
Here's a list of DAPs below $250, which support AAC and Opus playback, Bluetooth output, and 512Gb of expandable storage, and which are still available New online:
-HiBy M300, R2 II, R3 II,
-Shanling M0, M0 Pro, M1s, M2X
-Hidizs AP80 Pro-X