Time Machine for MacOS High Sierra - Delete middle backups only
You can do this with the Time Machine CLI: tmutil delete /path/to/backup

$ tmutil listbackups
/Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-04-142408
/Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-05-003504
/Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-06-005409
/Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-07-014517
/Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-08-023925
/Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-09-040035
/Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-10-043920
/Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-11-055340
/Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-12-063921
/Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-13-075323
/Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-14-085121

$ sudo tmutil delete /Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-10-043920
Deleting: /Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-10-043920
Deleted (149.0M): /Volumes/Mac SSD TM/Backups.backupdb/g/2018-01-10-043920
Total deleted: 149.0M