8GB should be fine for now. The waterfall/audio/data that are uploaded to network are saved temporarily in the RPi memory until their upload. The only reason to change that is if your upload speed is limited, however in such case it is better to use another storage solution as re-writing the sd-card will damage it sooner or later.
For the other questions around auto-scheduling I’ll bring here @ENSFNM answer from the matrix room:
You can set priorities in a file, more examples can be found in Satnogs-auto-scheduler priority examples