Group projects by Fiscal Year Date expression based on one field value, if field value is null then group projects based on another field value

Question

I am running a report where the projects are grouped by FY based on Fields!EstSubstantial_Completion.Value. Expression below:

=IIf(Month(Fields!EstSubstantial_Completion.Value)=10,
year(Fields!EstSubstantial_Completion.Value)+1,
IIf(Month(Fields!EstSubstantial_Completion.Value)=11,
year(Fields!EstSubstantial_Completion.Value)+1,
IIf(Month(Fields!EstSubstantial_Completion.Value)=12,
year(Fields!EstSubstantial_Completion.Value)+1,
year(Fields!EstSubstantial_Completion.Value))))

The expression is working, but my supervisor would like the projects to first be grouped into a FY based on another date field Fields!Savings_Report_Date.Value first and then if the field is blank (null) reference the Fields!EstSubtantial_Completion.Value field as the date to determine FY grouping.

I am new to SSRS reports so I am unsure if there is a way to write this type of expression.

Thanks!