This BananaPi RISC-V router board is officially supported in OpenWRT https://docs.banana-pi.org/en/BPI-RV2/BananaPi_BPI-RV2 and is very affordable. While not as powerful than the Spacemit K1 and having much less RAM, it works rather well and is very stable. Different metal cases available. POE option exist for the board as well.

Orange Pi RV2 is very similar in spec to this star9.com router.

Depends on the board.

Btw, I don't see anything about mainline in TFA, did I miss that?

FWIW there is also "OpenWrt mainline" and "Linux mainline"; OpenWrt carries a whole bunch of things on top of Linus' tree but I'd still call that "mainline".

Given the similarities in port layout (just missing a HDMI and USB3 header), and that the case is nearly identical, I would guess that this router probably is a custom run of the exact same BananaPi board without those headers. Both also use MiniPCIe in 2026, which is a bit of an odd decision.

The page linked above contains links to their bootloader and Linux kernel tree (6.1 apparently), so chances are rather low.