no, using another project with issuetype "Task" is no solution, because we're controling too much in Jira based on the issuetype. And nobody will change that manually on each new case.
At the moment, we use Jira actions in Watcher, beside the issuetype we currently also set priority, labels and environment from the fields available in the Jira action in Watcher.
Regarding UI, it must be possible to set the issuetype in the SIEM case, on the screen it could be placed near to the Tags field, would make sense to me. By the way, I could think of mapping the Tags field from the SIEM case to the Jira labels field. When I look at the SIEM Action API docs, this field is not mapped, right?
I could think of more useful functionality in the create-case action. The most important missing function for us would be the link-issue function from the Jira API. We use this a lot in Jira, actually we are linking new cases manually in Jira. This would be a big improvement for us in a future release update.