@gh0stface so this is what I've setup;
## S3 Storage
PF_ENABLE_CLOUD=true
FILESYSTEM_DRIVER=local
FILESYSTEM_CLOUD=s3
PF_LOCAL_AVATAR_TO_CLOUD=true
MEDIA_DELETE_LOCAL_AFTER_CLOUD=true
AWS_ACCESS_KEY_ID=xxx
AWS_SECRET_ACCESS_KEY=xxx
AWS_DEFAULT_REGION=xxx
AWS_BUCKET=mediapixelfedsg
AWS_URL=https://media.pixelfed.sg
AWS_ENDPOINT=https://xxx.backblazeb2.com
#AWS_USE_PATH_STYLE_ENDPOINT=false
and it works.