Dates Since last report

Since we're in the PQ forum, here is a solution with M. It looks a little crazy but should be relatively performant.

let
    Source = sample,
    NewColType = [Dates since last report=[Type=Int64.Type,Optional=false]],
    Sort = Table.Sort(Source,{{"Project Name", Order.Ascending}, {"Report Date", Order.Ascending}}),
    Group = Table.Group(
        Sort, {"Project Name"}, {{
            "rows", each [
                base = Table.Buffer( _ ), 
                new_rows = List.Generate( 
                    ()=>0,each _<Table.RowCount(base),each _+1, 
                    each let current_row = base{_} in 
                    current_row & Record.FromList(
                        if _ = 0 then {null}
                            else let last_row = base{_-1} in
                            {Duration.Days( current_row[Report Date] - last_row[Report Date] )}, 
                        { Record.FieldNames( NewColType ){0} } 
                    ) 
                ) 
            ] [ new_rows], 
            type list
        }}, 
        GroupKind.Local 
    ),
    ToTable = Table.FromRecords( 
        List.Combine( Group[rows] ), 
        type table Type.ForRecord( Type.RecordFields( Type.TableRow( Value.Type(Source) ) ) & NewColType, false)
    )
in
    ToTable

Tested with the below sample (generated with gemini):

Sample (excerpt)

Output