Which core ISS component's maintenance cycle now heavily influences the flight cadence of the Soyuz vehicle for Roscosmos?

The Zvezda service module, which remains a core component of the ISS structure.

The operational focus for Soyuz flights has shifted in the post-commercial crew era. While previously the cadence was largely dictated by the necessity of maintaining a full crew complement, usually requiring seats for international partners, the schedule is now more closely aligned with Roscosmos's internal rotation needs. Specifically, the maintenance cycle and operational scheduling related to the Zvezda service module, which constitutes a fundamental and core structural part of the International Space Station, are major factors driving when Soyuz missions are flown.