I thought it is possible using sequence feature.

But I couldn’t..

If on Facebook you can users opt-in to your messenger list to send a “broadcast” to all contacts that have opt-in to that list.

For Instagram you cannot send out a broadcast to all users. You can send a message once you receive a message from the user. But not to all contacts. 

ManyChat does have this feature but it is in Beta.

Hey ​@som!
As JJ said correctly, you can use the broadcast feature on all channel except Instagram to send a message to all you contacts.
Please be sure that you have their consent to contact them. Send message without prior consent is against Meta rules and GDPR.