Happy to help - I find as a support engineer (and more so as a mentor to our new colleagues) that we can sometimes get lost in the technical stuff, so spending some time on an adequate analogy is always time well spent.
While checking performance metrics from the VM/OS side is good practice, you should be checking the vSAN Client and Backend Performance statistics, these can be checked at the per-cluster and per-node level and in 7.0 U1 also at the per-VM and per-vmdk level - the Front-end/VM/Client level basically shows the speeds and feeds as it reaches the VM (in technical terms the vscsi layer), these can be accessed at Cluster/Host > Monitor > vSAN > Performance. In this case you should be validating that read latency and throughput is within reason for this type of cluster (e.g. while Hybrid clusters vary massively 5-10ms is likely reasonable) and relatively consistent e.g. no unexpected large spikes of 50-100ms, if this is a '9to5' cluster then relatively large spikes during morning and post-lunch boot/log-on is generally expected.
Regarding proactive rebalance - this is by design a slow and minimal process e.g. if there is 30% variance (the default threshold) between highest and lowest used disk, when you run it, it isn't going to try an make all the disks like ~1% variance disparity because 1. this won't always be possible (e.g. you just move where is imbalanced) and 2. unnecessarily moving data isn't always a good thing. If you are running this and it gets the highest-lowest disparity *just* slightly below the health alert trigger (30% disparity) then it will likely trigger again within days/weeks as whatever is on the higher-used disk is potentially growing faster than what is on the lowest-used. The fix for this is to update to 6.7 U3 where this is a toggle-switch option and it deals with this in the background (and in a more intelligent manner) without administrators having to manually start it. The option for now (before upgrading or if this is not possible) would be to use proactive rebalance via RVC where a lower variance threshold (e.g. 15-20%) can be applied.
Bob