If your snapshots are taken from your main subvolume, then they shouldn't be incremental backups, so it should be safe to delete ones from the middle. It does look like incremental backups are possible, though I've never used that feature: https://fedoramagazine.org/btrfs-snapshots-backup-incremental/
Two snapshots taken from the same subvolume would not be incremental. For them to be incremental, a snapshot would need to be copied from another snapshot. From that article I linked, it looks like you have you use the send command to actually make an incremental snapshot.