Have you considered hiring a trainer to work with her “resource guarding” and while looking for a doggy daycare who would be willing to work with you guys while she trains. Or better yet, a doggy daycare that offers training and who can work with her?
In the mean time, exercising her and wearing her out before you leave her for the day or after work will help. If she isn’t crated, offering enrichment/puzzle toys while you are gone can keep her occupied.
I do not recommend getting a submissive second dog to please your dominant first dog. It isn’t fair to the submissive and reinforces bad behavior in the dominant. Ideally you’d want two confident dogs on equal ground while you remain their alpha.