While I think it may be easy to argue that use of Matomo is legitimate for the purposes of improving your service (and as a corollary, this can legitimately be outsourced), I think it is a stretch to conclude that it is legitimate for the third party to assemble a dossier of an individual by tracking them across multiple sites for which analytics are outsourced to the third party.

This seems obvious since the third party is not going to expose details about the data subjects use of the other sites so it cannot be of legitimate interest to the primary controller. If they did share such data with the primary controller, the privacy violation is so egregious it cannot hope to pass a balancing test to be considered legitimate.

So while something like piwik may be allowed to be opt out, I think google may be treading on thin ice.