Hi @James Whitlock ,
You can deploy SharePoint Server Subscription Edition with Azure SQL Managed Instance (MI). SQL MI is a deployment option of Azure SQL Database and is compatible with the current version of SQL Server (on-premises), Enterprise Edition Database Engine. However, SharePoint Server farms must be hosted in Microsoft Azure to support Azure SQL Managed Instance. The SharePoint Server farm and the managed instance must be hosted in the same Azure region. SharePoint Server farms don’t support managed instances when hosted in customer data centers.
Regarding your concern about Workflow for SharePoint, I couldn’t find any information that suggests that Workflow for SharePoint does not support the use of Azure Managed SQL instances. Please pay attention to Microsoft's official website to obtain the latest information on this matter. I will continue to pay attention and any news will be shared here in a timely manner.
For your reference:
Best practices for SQL Server in a SharePoint Server farm
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