Which router model are you using, depending on the model, you may have some device specifids throughput pusage. For example, on many Netgear rotuers, if you have QOS enabled, the attached devices page will list the per current throughput usage, though it will not maintain a count of the overall data use over time.
While it will not be all of the information you want, it would be able to tell you what is using a bunch of data, especially if you monitor while the devices should be idle.
It is also useful for spotting a compromesed device that mat be taking part in a bot network.
Beyond that, if it turns out to be streaming, see if you can fnd ways to save on data, e.g., instead of streaming, consider checking if you can find any h.265 versions of the content that you can store locally, especially for content that multiple people in the home may want to watch.
Also keep an eye on some of your game launchers, some such as the microsoft ones use really bad DRM where often entire encrypted packages need to be replaced, thus what may be a small 100MB update on one platform, may end up being a 10+ GB download on the microsoft store for the PC due to their DRM not allowing certain archives to be modified.
Also, certain free to play game launchers such as the one for armored warface, use the DHT network for distribution and can result in the client also seeding updates for others (make sure you are not seeding).
