In multi journal setups, you get breadcrumbs per journal. So if I am visiting an issue page, clicking on “Home” will only bring me back to the journal’s home, not the “main” index with the overview of all hosted journals.
What would be the smartest way to add such a link to breadcrumbs or to the navigation?
Presuming that you’re using OJS 3.3 (you didn’t indicate your specific version [e.g. 3.3.0-11] - if you could do that in the future, that would be helpful), you could include a navigation option to navigate to your main site: https://docs.pkp.sfu.ca/learning-ojs/en/settings-website#navigation
Yes, I think you’d have to in this case, as it doesn’t look that linking back to the root journal site is an option among the existing navigation options.