Steve Motton "...also how are image updates to an existing pool handled..." The following may help....
https://www.youtube.com/watch?v=2LxvwR9LGWQ&list=PL-V4YVm6AmwXGvQ46W8mHkpvm6S5IIitK&index=24
Windows Virtual Desktop - #23 - Update Session Hosts from Latest Image
Learn how to use the Shared Image Gallery to update your WVD Session Hosts from the latest image today at The Azure Academy. Shared Image Gallery is a service that helps you build structure and organization around your managed images. Using a Shared Image Gallery you can share your images to different users, service principals, or AD groups within your organization. Shared images can be replicated to multiple regions, for quicker scaling of your WVD deployments.
As well as this coming soon: https://azure.microsoft.com/en-gb/updates/azure-vm-image-builder-service-will-be-generally-available-june-2020/