Is it possible to change the ItemID of a Portal Item (e.g. a webmap) to a specific value, or to set a specific ItemID when creating a new item? I don't see any API supporting that so I assume it isn't possible...is my assumption correct?
You are correct, it is not possible to specify an itemID or modify an existing one, although I do see some potential value in doing so for certain use cases.
If it does not already exist, I would recommend posting an idea for this here on GeoNet.
Here is an update
/addItem: Add Item—ArcGIS REST APIs | ArcGIS Developers
New at 10.8.1. The item ID of the item. The item ID must not exist and must be 32 alphanumeric values.
itemIdToCreate applies only to ArcGIS Enterprise.
Allows to specify the ItemID to Create: