Have a look at the FileShareStorageLocation parameter
You can tell SSRS to store the temp/swapfiles somewhere else

The documentation mentions the windows failover cluster manager
with screenshots

Time for a coffee break.

Sorry polluting sql 2017 administration. Why can't migrating SSRS to 2017 be simple?

It didn't like the VirtualServiceAccount it was running under;
Changed to an active directory account and it connected

Check the pro's and con's of 2017 implementation ( around july 2017)

From memory
1. Can you connect to the windows 2012 vm remotely? The vm might use an internal network.
2. SQL Configuration Manager: is TCP protocol enabled?
3. The SQL...

Source control is important so you can rollback to a previous version.
c# pro:
no license of SSIS needed

flexibility
c# contra:
requires c# skills
no visual flow control

Searched the web a little further. Refs not supported on san-storage? 
Not yet recommeded for clustered volumes due "redirected mode" ( Redirected mode: can't bypass...

Refs is supported but I can't find more information. Have you tried it in a test-environment?
Refs seems to have a memory impact

SQL will likely be faster as PL/SQL causes cpu-context-switches.
Perhaps they made cursors for the error-handling?

Disabling reportlogin and killing reportsessions sounds feasible.
Another option is read committed snapshot isolation ( depends on what kind of ETL is running)

Can't really recommend further steps, as you checked a lot.
Anything odd on the VM(host) side? Like not running in high-performance mode, overcommitment, old drivers ...
Maybe ssrs is...

Hello,
If you can't optimize the reports Troubleshooting Reports: Report Performance
you might want to check SSRS subscriptions under the hood
It mentions MaxQueueThreads in the...

I don't have a compare case Hyper-V versus VMWare
You might want to check the setup of the Hyper-V host , the Hyper-VM , cpu, network, attached storage
MS realises...