Is there a way to create the bpmn:ServiceTasks, or any other special forms of the task element straight from the palette?
yes, there are ways to do it. I’ve created a modeler plugin a long time ago that can serve as an example: GitHub - ingorichtsmeier/camunda-modeler-plugin-connected-elements: Example of a modeller plugin to add some connected elements by dragging them as a single symbol from the palette.
Disclaimer: I haven’t tested it with the latest modeler version.
Hope this helps, Ingo