It isn't posible to set an alternative Domain for Preview
Since #20801 the first domainrecord is always used for the Frontend-Preview. It isn't possible to overwrite this or to define another Domain Record as the Preview-Domain. In some reasons you wont't have the backend accessible over the Main-Domain e.g. security issues.
By switching the order of the domainrecords you get some other issues with functions or extensions, when they use the first domainrecord for the frontend rendering too.
It would be nice to overwrite the Preview-Domain in the PAGE-TSConfig (also possible for multi-domain sites) or deactivate special domainrecord for Frontend-Preview.