1. get a brand new SD card and flash the recovery image as instructed 2. now you can boot on it least once so the home partition (the last ext4 one) expands to the entirety of the card, or maybe you want to manually do that with your favourite partition editor 3. then you can drop a steamapps folder in the root of that partition owned by the correct user (you probably don't even have to chown it if you're UID 1000), and also the libraryfolders.vdf file too, so you can put your games on it and it correctly gets recognized instead of asking you to format it in the storage settings (don't format it)

probably possible by overwriting all the partitions except the last one