On 21 January 2023, Randers FC confirmed that the club had signed 18-year old Kaastrup from F.C.
[citation needed] After a few appearances for Randers' U19 team, Kaastrup made his official debut for his new club on 23 April 2023 against Brøndby IF, replacing Adam Andersson for the last few minutes.
[4] In pursuit of more playing time, on 31 January 2024, Kaastrup was loaned to Swedish Allsvenskan club Västerås until the end of the year.
[5] However, Kaastrup did not play any official matches in Västerås before returning to Denmark in the summer of 2024, and on 2 September 2024 his contract with Randers was also terminated.
[6] On September 12, 2024 Kaastrup moved to Danish 1st Division club FC Roskilde on a deal until June 2025.