A potential project for Tiki26 or Tiki27 is to add a recurring billing feature as part of the revamp of the accounting feature, using System Features (system trackers mainly)
If you need this feature, please contact Marc Laporte.
Options
hiqdev/php-billing
https://github.com/hiqdev/php-billing (BSD-3 license, part of HiPanel: https://hiqdev.com/packages/hipanel/)