Hmm, how about go "all-in" with the text-only approach and dispense with the database backend as well (use the filesystem-as-database). In a sense focus on the absolutely minimum resources and layers, avoid any moving parts and fragility and deliver the basic activitypub federation experience but in a very solid way?

Would this be attractive to more people than a niche? Not clear, but coupled with a performant language it might be "blazing fast" while very economical.