It could be that steamlink is using proprietary acceleration bits specific for the Raspberry Pi boards. Not sure if steamlink is an open source project with visibility into what the code is doing. If it is, it can be modified to support standard interfaces.
Did you ever end up getting this to work? From my understanding arm64 chips need to use the moonlight-embedded to work correctly. I’ve tried almost all the distros recommended by libre but always run into errors. I’ve finally found out that the embedded version is the only one working with arm chips but still haven’t been able to get it work.
Took a look at the code for moonlight. It seems they are using the legacy Amlogic SDK video interface rather than the upstream V4L2 M2M interface. Someone will have to write the code for moonlight to use the upstream interface.