Numbers are in after two weeks. Keepalive 300 (our carrier is fine with it, no timeouts in the Log tab), batches of 3, attributes only on change. Projected ~9 MB per SIM per month, down from 25. The attribute chatter was the surprise: our firmware was publishing lan.ip and cell.rssi as attributes with every reading. Moved rssi into telemetry every 15 minutes instead. Thanks Kevin, those measurements saved me a week.
Timestamps in UTC or it didn't happen.