I did convince all the people I was close enough to and with whom I was in a common group by saying them I left Whatsapp for privacy and ethical issues, but if they wanted, we could juste continue the conversation of this group on the similar one I just created on Signal + link of the group.

It worked very well (high school friends group, family group, college group, hobby group...).

Of course you need a partner who is already on Signal for each Whatsapp group, just to be a able to create the new group on Signal 😉