Maybe this app is something for you?
https://github.com/itsdrnoob/DataMonitor
It can be downloaded as an .apk from Github or installed via Google Play or F-Droid.
You also seem to be able to create widgets for monitoring data usage with it.
Just for the record: I haven't tested the app myself as for me personally, the path via Settings> Network & internet> SIMs> your SIM> App data usage
is sufficient.