Not everything I have here is FOSS. But theses are my favorite alternatives thus far to some of the Apple ecosystem apps.
Safari —> Librewolf
iCloud —> Mega
Notes —> UpNote
Your question prompted me to think about Proton as a substitute for Calendar. And this made me think about Proton as a substitute for i loud as well. As a web app, Proton could serve as a cross platform solution for mail, calendar and cloud.
Zulip is a FOSS alternative for Slack and goes across all platforms. If you’re willing to adjust some of your workflows, Zulip could end up quite useful.