I would also suggest to change the config a little bit to be able to serve first party cookie. This also might solve your problem, if you can’t fix it otherwise maybe worth to try.
Let’s say, that the landing page in question is www.yourlandingdomain.com, and your mautic is hosted at mauticdomain.com.
You can add a CNAME with a new subdomain for t.yourlandingdomain.com pointing at your mauticdomain.com domain.
You have to add t.yourlandigdomain.com as an alias to your hosting pointing at mauticdomain.com as well.
You can test it -> if t.yourlandingdomain.com opens your mautic, you are good to go.
Then the last step is to change your scripts using the t.yourlandingdomain.com parameter.
This also solves your problem with a poosible cookiecalypse: https://econsultancy.com/marketing-guide-third-party-tracking-cookies-timeline-alternatives/