I marked all my project on github as closed and added a link to the codeberg one.

After years google will still link to github anyway.

I had thought of just keeping them open and sync them, but I thought in that case people would presume the project is actually active on github and try to interact creating issues and pull requests and getting frustrated.

I must say codeberg's reliability isn't the best, but with all the scrapers that hit them it's a miracle they do seem to manage. And at work things on our private network without scrapers aren't more reliable.