Munich of all places offers an interesting case study on open source software in German public service. They ditched MS 21 years ago despite Bill Gates' personal lobbying of mayor Christian Ude. But they had to go back eventually because they simply couldn't make the alternative (based on a custom Linux implementation with Libre Office) work. They've now shifted to using open source as much as possible without sacrificing interoperability.

This page has a great writeup on the topic: https://interoperable-europe.ec.europa.eu/collection/open-source-observatory-osor/document/munichs-long-history-open-source-public-administration