That surprisingly wasn't that hard, actually.

One side of the family already settled on Threema for communication years ago, the other side I had to first show Signal to and help with installing it, but now they are even using it in other contexts.

And apparently I live in a bubble that is quite open to Signal, so most of the people I know at least use it in parallel to WhatsApp. And for the rest there is still RCS.

The actual hard part are (since I became a parent) all the WhatsApp groups that exist for most kinds of organized child activity, including distributing much of the crucial information regarding school.
Also at my wife's work WhatsApp is standard for coordinating with the colleagues.

For that purposes we use an old customized WhatsApp-only smartphone, that is used by the family like some old-fashioned fixed phone-terminal, only for messaging...