Be aware your backup is useless, if you don't backup nextclouds database when using a bucket as primary storage ☝🏻
I use nextcloud with local storage and I use rclone for backup to a S3 bucket (MinIO).
You can use rclone to directly backing up your bucket. There is no need for restic. It is simple like rsync
An example would be:
rclone sync b2:mybucket otherprovider:otherbucket
You can use local storage too:
rclone sync b2:mybucket /my/path/here
Rclone can mount and backup almost everything. It is a swiss knife and I love it.
Performance is really good with nextcloud, if configurated well. Look at this thread