I think this isn't necessarily about open source, it's about tracking users in order to manipulate them with relevant ads. A company could use a fully open-source stack and still cross that creepy threshold with user tracking no problem.

A few years back I was using a paid Google Apps account (which IIRC excludes you from some tracking?) and FastMail simultaneously. Both were excellent from a technical perspective, but FastMail was just better. They make all their money from selling their service to end users and I trust this model more. So I switched to using FastMail exclusively.

I use free and open source software and strongly prefer it - but I don't see it as some kind of 'automagic' solution to tracking and privacy concerns. Buying the product, and not being the product, is what is important to me.

Also don't forget to self-host what you can, for example it's not that difficult or expensive to set up Nextcloud on a VPS for file and contacts/calendar syncing. And don't be a cheap bastard with the FOSS you use, make donations to developers of useful tools. You're going to pay for all this one way or another.