for anyone reading this, Codeberg/Forgejo can migrate issues too! use the "new migration" function in the menu where you create new repositories, and tick the box for copying issues and wiki. it is a one time copy only, though, so if you are dedicated you should restrict issues on the github repo to collaborators only, so that people can't open new issues (which won't be able to be synced anymore), but old ones are still readable in their original form.
syncing issues cannot be done later, it's for new repos only