Whatsapp uses a variety of XMPP to communicate, and people have figured out how to do XMPP<->Whatsapp bridges. See:
You can probably find a XMPP server that has yowsup2 and transWhat installed so you can use the Librem 5’s Chatty (which is a XMPP client) to communicate with Whatsapp, or you can install these programs yourself in the Librem 5. I haven’t tried it myself, so maybe I’m overlooking something, but some people have gotten it to work.
By the way, you can also replace Outlook with the Geary email app.