Hi Kai,
in principle it is advised to create Pluggable Widgets and only resort to creating new Custom widgets if really needed. The type="entity” setting in the widget XML is particularly available for Custom Widgets, not for Pluggable Widgets, which are typically loading data either via accessing an attribute of the data view the widget is in via type="attribute” or via accessing other data via widget XML type=”datasource".
Can you go for the Pluggable Widgets instead?