You should define the term set directly in the Term Store Management in the Managed Metadata Service Application using the Central Administration (if you have the necessary administrative rights). There you can define the term set accordingly and globally so that it is available throughout the farm.
In the individual site collections, you will then have to pin the terms, as described in this blog post.
This method does have another drawback in addition to the manual pinning: When you change the URL for a term in the primary term set (defined in Central Administration), the term in the secondary term sets (in the site collections) will not be updated automatically. This could be handled by running a PowerShell script after the navigation was changed, but i haven't tested that yet.