As others have mentioned Vault, Keycloak, Flink are language agnostic. Regarding the switch from Java to .NET, I would rather recommend switching to Kotlin instead of .NET for a developer experience similar to C#, while still keeping your existing expertise in Java and its ecosystem. And this comes from someone in a .NET shop currently, but have worked with Java before. IMHO both languages and surrounding ecosystems are good. Both have their pros and cons and quirks.