Some older portable media players are supported by Rockbox, which supports Opus. Unclear if there's any supported devices currently in production. As for SD cards, I know SanDisk devices on that list supported microSD cards.
I put it on my Sandisk E200 back in the day; 2Gb and then I stuck in another 2Gb card. It was so responsive, and the scroll-wheel UI was so convenient. The wheel seized up (still usable, but had to turn it like a safecracker instead of with your thumb), so I gifted it to a co-worker's son.
What I always wanted to see was something with an "old Blackberry" style side-mounted scroll-wheel, for eyes-off operation.
I can't seem to find any PCB-mounted jogwheels that both (a) rotate in a plane parallel to the board and (b) are meant to be operated by touching the edge of the wheel (rather than its face, like the dial on the Sansa Fuze) and (c) don't cause the depth of the device along the axis perpendicular to the PCB to become huge (i.e. like a knob-style encoder without the knob).
I can't remember how battery life compared for MP3/AAC (was RockBox able to use the hardware decoder?).
https://www.linsoul.com/products/xduoo-x3ii
I wish it were the size of a Clip+.