Be aware that, as part of its security concept, Android runs every app under a different UID. As a consequence, with an ext4 FS, data sharing across apps can be difficult unless they use the common GIDs (media_rw, etc.) foreseen to that effect. In the past, many people used FAT-formatted SD-cards to circumvent that restriction, taking advantage of the fact that FAT simply does not know about UNIX file permissions. I do not know if this affects your use case(s).