Getting a connected report after installing carrier services was likely a spurious correlation, as it's now back to 3000 status and crashing GM when trying to report the connection state via debug menu. GM is installed via play store, the SIM is freshly provisioned and all settings are at their defaults after reinstalling to test the stock OS.
If this is an issue with throttling or something on the carrier end, why does it verify and provision instantly on a stock OS with the same configuration? Something with GOS is evidently still tripping an integrity verification mechanism or failing to pass through required data.
Going to look through the logcat entries in a bit to see if I can spot anything noteworthy.