@Strategos, thank you for posting this issue on Microsoft Q&A.
After configuring the DCRs, it may take a while for the inventory to be uploaded. Based on the DCR template, which is used to create the DCR for enabling change tracking at scale, the inventoryCollection frequency is 36000 seconds:
"inventorySettings": { "inventoryCollectionFrequency": 36000 },
This would mean every 10 hours.
Therefore, please allow it sometime and check again if the software inventory is being collected or not.