For future records SCCM Ingestion looks like a good option. Also if not OOTB SCCM Integration, you can also explore JDBC based SCCM custom integration / data source.

For License calculation, do you have any documentation around how ServiceNow is calculating the records, what filters being used? Some tweaks to certain attributes of existing records so they are excluded from calculation could also be a more viable option. For e.g. if ServiceNow considers records in specific lifecycle state for calculation or Most Recent discovered records from last 1 year, you can simply tweak those fields of existing records to get them excluded from counting towards licenses.