However, doing this and using a custom ROM will flat out disable SIM services menu and related functionality, which I need for my embedded e-Signature in my SIM card.

Also, we have a gatekeeper called Google Play Services. In other words, even if you have a custom ROM, you need that all-encompassing framework to have nice things.

Clients may differ, but the cloud platform and the connector which enables all these "Android" things is closed source, and controlled by Google.