Is there a recurring fee 💸 of Synology licenses for software maintenance?

It depends. Some are one-time-cost with lifetime update. Some are not.

Details are listed below.

Free for Surveillance and MailPlus Licenses: 

These licenses are truly lifetime valid, offering perpetual access to all future updates and features. Migrating them to your future Synology NAS is easy. 

Upgraded Fee Required for Virtual DSM License: 

Enjoy 3 years of worry-free upgrades with this license. Continue using the existing software afterwards, but new features will require a renewal.

Upgraded Fee Required for Virtual Machine Manager Pro licenses:

Choose your control period (1, 3, or 5 years) for managing your virtual machines. After expiry, existing VMs remain functional, but creating new ones requires renewal.