IDMarkz is a Mac utility from Markzware for converting InDesign files (to the IDML interchange file format, annotatable PDFs and other formats) for use in Affinity Publisher (1.8+), QuarkXPress, older Adobe InDesign versions and more, without requiring the Adobe subscription software itself. Other features include the ability to preview InDesign documents, automatic conversion and opening in your chosen app, AppleScript support, and an inspector to show document details.
IDMarkz is priced at $199 for macOS 10.12 and later. Coupon code IDMarkzLaunch provides a 50% discount through the end of the year. (A Preview app, free in return for providing an email address, allows viewing InDesign files but not exporting them.)