How to have matomo checkbox checked by default ?

I'm sorry, but this would break the rules of the GDPR what make a tool like this modul necessary. It requires that the users actively decide (by checking the Matomo checkbox, or accept all 3rd party services).
I think it's great that you're using Matomo instead of commercial tools, but it's still a tracking tool. I hope this argument convinces your customers to forgo it. Protecting private data is in all of our interests.