What apps you install depends on your needs and preferences. It might help others if you include those in any future requests for suggestions.

I suggest not worrying too much about "removing your dependence on closed ecosystems" immediately. Just do as you did before, changing apps as you find better alternatives---only, this time, considering the advantages of FOSS. Simply by giving F-droid apps a chance before opening (I assume) the Play Store, you're already doing better than the vast majority of people.

Regarding discoverability and security, I believe participation in the community helps:

• The Venn diagram of "FOSS app users" and "software enthusiasts" is closer to a circle. People like talking about useful, good software they like. Word of mouth recommendations is how I got into this stuff.
• You'll be more likely to hear urgent actionable news (e.g. X app developer sold to bad company, here's the fork that will carry the torch onwards).

And so that this comment isn't completely useless... Mozilla are currently working on a mobile version of Thunderbird for Android, built on top of K9 mail. Been using the beta and liking it so far. If you want a FOSS e-mail app, keep an eye on that one.

P.S. I much prefer the dark side, and don't forget the cookies!