get a table of all tables across x number of log analytics workspaces

cryptoSHA 

You need a Cross Workspace Query, please read Query across resources with Azure Monitor - Azure Monitor | Microsoft Docs

The Usage table is optimised to gather this data, and its cross workspace friendly -  One example is: 

workspace("yourWorkspaceName").Usage
| where TimeGenerated > ago(1d)
| summarize  SizeMB = sum(Quantity), SizeGB = sum(Quantity)/1000 by DataType, IsBillable

 You can extend this and use Pivot mode to display the results

union 
(Usage
| where TimeGenerated > ago(1d)
| summarize  SizeMB = sum(Quantity), SizeGB = sum(Quantity)/1000 by DataType, IsBillable, workspaceName='local'
),
(
workspace("nnnnn").Usage
| where TimeGenerated > ago(1d)
| summarize  SizeMB = sum(Quantity), SizeGB = sum(Quantity)/1000 by DataType, IsBillable, workspaceName='fake'
)

If you know about Azure Workbooks, that has a feature where you can run a Query against any selected Workspace. An example if you want to go and look at the process, but its will only show data if you have Sentinel  Azure-Sentinel/SentinelCentral.json at master · Azure/Azure-Sentinel (github.com)