Encrypt all the things!

Not really.

You can take it as three scenarios:

1. You actively use WhatsApp and you keep all existing conversations = they build a profile off the old conversations, match to any FB profile, and then enrich it going forward.
   
2. You use Signal in future and you keep existing conversations in WhatsApp = same as above but they cannot continue enriching the profile going forward
   
3. You use Signal in future and you delete everything you can = you minimise the profile they can build (if they haven't actually already done so)
   
   

All choices are valid, they're your choice.

I'm somewhere between #2 and #3 depending on which conversation, which contact.