The idea of copyright is to prohibit unauthorized use and reproduction, but none of this actually happens with a proprietary software SaaS backend. You don't actually give anybody the code - they connect to the service.

Access to the service is already governed by computer access laws, which don't depend on copyright. And if you never intentionally distributed your code outside of your org, you can call it a trade secret and nobody else has any legitimate right to access it - whether or not it is copyrightable.

There are other things that aren't copyrightable that are trade secrets already. This would be true of any kind of automated data collection for example. You couldn't copyright it but you can call it a trade secret.

And for any of that stuff, if you want to share it and limit distribution, you just have whoever wants access explicitly agree to be bound by contract law.