I finally managed to realize this (obviously the need was not that high looking at the amount of time since the OP 
).
Basically when using HA OS there needs to be ANY workaround to make this work reliably. The best one with fewest downsides I could find and tend to recommend is using the community SSH addon as tool/information source. Two implementation options:
- writing to file: Unreliable InfluxDB size sensor - #30 by tom_l
- writing to MQTT: Unreliable InfluxDB size sensor - #55 by beylie
There are a few little things to look at (e. g. Unreliable InfluxDB size sensor - #57 by e-raser) but it seems to work quite good. Information flow in generel is:
- SSH addon
file/MQTT broker- reading from 2 and creating a sensor
A lot of useful information on those two options (and other implementations/possibilities) are in that already recommended topic.