1⃣ Google Calendar: neither #Nextcloud nor #GNOME Calendar correctly handle notifications! https://github.com/nextcloud/calendar/issues/3000, https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/498.
GCal famously does not support CalDav authentication https://support.google.com/calendar/thread/55961951/can-i-subscribe-a-caldav-calender?hl=en, which makes migration really difficult.
2⃣ Google Maps: Now juggling #OrganicMaps + Citymapper + waze (the latter owned by Alphabet anyway). Organic Maps is still a big lacking & buggy: many UX issues, navigation a bit choppy https://github.com/organicmaps/organicmaps/issues/1060.
This is a problem that I have as well! The network effects and the low barriers of entrance for new contributors (at least for those who already have a Github account) are really valuable.
For this problem I hope that some "federated Git" system will help. Apparently there is development under way e.g. for Gitea: https://nlnet.nl/project/Gitea/ , so there is hope.
@astrojuanlu try Magic Earth for maps. Navigation is good, there are good voices in several languages, it uses open street map data, and visually, the default is better than osmand and organic maps (the latter two are highly customizable, though). Its not open source, but they appear to be privacy centric.
@astrojuanlu For CalDAV I'm quite happy with Thunderbird on the PC, and with DAVx⁵ (https://www.davx5.com/) + Etar (https://f-droid.org/de/packages/ws.xsoh.etar/) on Android. Both solutions also support notifications.
(not sure if you were actually looking for comments on your concrete obstacles - feel free to ignore my unsolicited advice :)
@ollibaba Thanks for chiming in! I'm not happy with Thunderbird though, every time I try it it sets my CPUs on fire https://bugzilla.mozilla.org/show_bug.cgi?id=1753282
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