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.
Zorin is a choice. It provides security, custimability, stability, active development and improvement, ownership, privacy and a community of helpful and knowledgeable like- minded individuals. There is no big comp agenda or data farming. Like many versions of GNU/Linux, it offers you the power to take control of the hardware you purchased and do what you need and want it to do. The difference with Zorin is that it gives you an interface, the choice of an interface, you are familiar and comfortable using. For those that use it daily, it feels like home (one you can remodel as you learn, grow and adapt in life).
On my old Acer (quad core machine with 8GB of RAM) I tried ElementaryOS. It wasn't a week later I stumbled upon Zorin. It brought life to that machine (with a few required tweaks to get all the features working). It was fun, familiar, and with a little work, configurable any way I desired. I purchased a brand new development/gaming laptop. I was having issues getting things to work on 15.3... then I beta tested 16. Everything worked OOTB. That allowed me to focus on software and customization, trying XFCE, Cinnamon and finally settling on Plasma. I code python and javascript on my dual boot machine (Windows is strictly for gaming.... it has no other purpose). My daily driver is Zorin. It has met every requirement I have to use it personally and professionally. VSCode, IDEA Community Edition and Android Studio all run beautifully. Libre Office has replaced M$ Office, Evolution handles my many email accounts. I couldn't see going back to windows or even considering Mac. Why, when this OS has given me everything I could want or need... and really allowed me to own my computer. No registry tweaks, hacks and stripping the OS to prevent data farming.
Hackgets said it best... try it, compare for yourself... if you don't think so, that's alright. Find the distro that feels comfortable for you. The moderators, tech support and regular users here, the ones that help others to resolve their issues, we found our home (most in Zorin, a few in other distros) . We want to help others find theirs, whether here or in another distro. But we're here because we're passionate about Zorin and our community.