Hi James - there is an assignment rule driving the behavior you are seeing here (i.e. SIRs being auto assigned to the SIRT group).
The Assignment Rule here is on the <sn_si_incident> table that needs to be modified. It is called "SIRT Assignment" - refer to screenshot.
If you change this Assignment Rule to Active = False, you can achieve the behaviour you are looking for by creating a new Assignment Rule with your tailored requirement (i.e. default assignment group for newly created SIRs). You can use the baseline "SIRT Assignment Rule" as an example / reference, when you create your new Assignment Rule specific to your requirement.
Previous post related to this also found here: https://community.servicenow.com/community?id=community_question&sys_id=ab3673e0db8f5384200f0b55ca9619e9&view_source=searchResult
