After seeing a #postmarketOS merge request adding the Xiaomi Pad 5 non-Pro (xiaomi-nabu) and seeing they had the pen working, I cherry-picked some commits from the sm8150 kernel tree to the sm8250 kernel tree and got pen working on the Xiaomi Pad 5 Pro (xiaomi-elish)! I created a merge request against the sm8250-mainline project so hopefully it gets pulled into postmarketOS in the next kernel release.