Alright, thanks to someone from the Forge Discord I managed to solve the issue.
If anyone ever gets this error: java.lang.RuntimeException: java.lang.StringIndexOutOfBounds: begin 0, end -1, length 3
Delete the client-extra.jar, found under external libraries, and refresh your gradle.
Also make sure you are using Java 16 (As of 1.17+)
This solved the issue for me, which took me days.