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davx5-ose
DAVx⁵ is an open-source CalDAV/CardDAV suite and sync app for Android. You can also access your online files (WebDAV) with it.
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WorkOS
The modern identity platform for B2B SaaS. The APIs are flexible and easy-to-use, supporting authentication, user identity, and complex enterprise features like SSO and SCIM provisioning.
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icsx5
ICSx⁵ is an Android app to subscribe to remote or local iCalendar files (like time tables of your school/university or event files of your sports team).
you may want to take a look at https://grapheneos.org/install/web before you come to that conclusion. it's so much more easier now. plus ProtonAOSP developer was the person behind developing that web based installer. checkout https://github.com/kdrag0n/android-webinstall for more info
Pretty much the only choice we have is to flash a custom rom without Google Mobile Services and GApps, while hosting a private Nextcloud instance and use DAVx⁵ + ICSx⁵ for CalDAV (Calendar), CardDAV (Tasks) and WebDAV (Cloud) based private mobile services. This sets the technical bar pretty high.
Pretty much the only choice we have is to flash a custom rom without Google Mobile Services and GApps, while hosting a private Nextcloud instance and use DAVx⁵ + ICSx⁵ for CalDAV (Calendar), CardDAV (Tasks) and WebDAV (Cloud) based private mobile services. This sets the technical bar pretty high.