I would suggest using either GitHub or GitLab as primary and mirror on the other. I prefer the issue management of GitLab so use that as my primary.
The top search result[0] are GitLab docs for pushing to or pulling from another repository for mirroring.
How about setting up your own server and use https://github.com/go-gitea/gitea to host your projects there?
>I don't want to host them locally. Too much of a hassle.
No it isn't. Just buy an external drive or something and copy your repos to it, done and done.