Defender Williamson, who made eight appearances and scored one goal during the 2018-19 season with Boro, and forward Bacon, who made five appearances for the Seadogs in the same campaign, are among the five players retained by boss Curtis Woodhouse for the 2020-21 season.
Former Bridlington Town left-back Jack Walters has also secured a new deal with Trinity, along with forward Gregg Smith and midfielder Alex Wiles.