The problem does not seem simple and easy because RTZ route files exported by ECDIS are very complex and contain a lot of information. This is probably the reason why they are not directly and easily convertible into simple plot files like GPX, XML, etc.
A previous thread
https://community.windy.com/topic/25467/routes-import-from-ecdis?_=1714803531545
Paths for developers ?
https://stm-stmvalidation.s3.eu-west-1.amazonaws.com/uploads/20190322122013/ECDIS-Annex-S-version-1.1-STM-Extended_29032017_Final_Proposal.pdf
https://www.cirm.org/rtz/