the only other way to do it on historic records would be to use sys_audit and focus on the elements of assigned_to and assignment_group
a query such as this would give you last 90 days:
https://xxx.service-now.com/sys_audit_list.do?sysparm_query=fieldname%3Dassigned_to%5EORfieldname%3D...
The other way might be to use the method I talk about here and generate sys_history sets for your records then build a report on sys history for incidents, the difference is the values returned in there are much nicer than the sys_ids your going to get from sys_audit
Reporting on CI Changes - Using History Sets
One word of caution when dealing with either is if you've got a very large instance, this is going to take a while and could even cause some degradation whilst your transaction is hogging resources, so I would try to focus on specific dates, fields and do a fair bit of testing in sub-prod instances.
Hope some of that helps anyhow.
