[url=https://www.nookgaming.com/harukuru-spring-has-come-true-r18-release-confirmed-r17-out-now/]Blog posts[/url] from Sekai Project and Denpasoft (both linked on source) indicates that the adult version is coming as a standalone release later.
No R18 DLC is to be available for the Steam version.
Status (2025-11-10): In QA