The method the other commenter suggested also works.
But yes, these are anecdotal for WhatsApp, which I presume was just an example on your side.
You'd have to mainly use F-Droid and then make some exceptions for those apps. You could also use Yalp store (which interfaces with the Play Store) or Aptoide to get those apps, if you don't want to keep the Google Play Services around, though I cannot make any claims of these being more secure.
Your question demands an answer, for real. As does mine.
There has to be a middle ground. In this case, unfortunately, besides close friends or family who know something about tech, you will have to be the one to compromise and use their apps. Plus, I've seen getting people to move to your communication methods which are less popular usually leads to less talking. Not always, just something I've noticed isn't uncommon. I end up talking to people more if we just use FBM or iMessage or something.
But if you need to download WhatsApp to communicate with loved ones, you can't get it from F-Droid, right?