Former Sutton United playmaker Harlow had been brought to the Broadfield stadium at the start of the season, with Smith hoping his experience would help Reds push for the title.
However, despite an impressive opening half of the season, the veteran found himself surplus to requirements and had to settle for a bench place in the latter stages.
The departure of Dack has been on the cards as the left-sided midfielder has failed to reproduce anything like the form he showed in his first spell with the club.
Ugbah has been out for the majority of the season through injury, while Restarick, despite showing 100% effort when picked, has been overtaken by Reds' younger strikers and increasingly found it difficult to command a regular first-team place.
It looks likely now that the 29 year-old will make his switch to local rivals Hastings Town a permanent one, after a successful loan period.
Meanwhile, Smith has snapped up ex-youth teamer Ellis Hooper from Horsham YMCA on a one-year contract.