Dear @Retired_mod ,
as You mentioned, Databricks does not provide out of the box support for VM usage monitoring for job clusters created from cluster pool.
If we really want to use cluster pool, I would consider:
1) splitting the pool into separate pools on the granularity level that reflects our needs for costs reporting,
2) extrapolate the DBU costs to VM usage costs (as they are very similar) on the job level,
3) tune pools (setting min idle, autoterminate) to minimalize idle time.
An alternative approach would be to use job computes - not from the cluster pool, and then use tagging.