Maintaining and deploying many multi-site from a single codebase
Craft's Project config is really made for updating changes from staging to production.
When it comes to making changes across a fleet of (different) sites, you're somewhat on your own there. You might think of all of your sites as the same but Craft is thinking of each site independently because that's how it's designed.
It sounds to me that one way to handle this might be to try to automate migrations between installs? You change one site, test it, and then move the migration to the next client site/grouping.
After all, if you're changing or adding fields, for example,  you're most likely going to be changing the (GraphQL) code that also queries those fields as well or everything is going to fall down pretty quick.
Something like Migration Assistant could useful here?
Also, I'm not sure how your devops is setup but theoretically you could share code between installs (same templates, plugins, etc.) The vendor, migrations,  templates, and other code  directories could be symlinked in from a central spot so when they get updated, the rest of the sites get updated as well. Once the build process is complete you could fire off a craft migrate/up command line that propagates across each install to make sure everything updates.