Managed to solve it with this bit of SQL I wrote. Only works on SSMS 2019+

DECLARE @MyCursor CURSOR; DECLARE @ItemID int; DECLARE @ItemTextValue nvarchar(max); DECLARE @ItemValid int;

BEGIN SET @MyCursor = CURSOR FOR SELECT pd.id, textValue FROM umbracoPropertyData pd INNER JOIN cmsPropertyType pt ON pt.id = pd.propertytypeid INNER JOIN [umbracoContentVersion] cv on cv.id = pd.versionId WHERE ISNULL(CAST(pd.textValue as nvarchar(max)), '') != '' AND dataTypeId in (SELECT [nodeId] FROM [dbo].[umbracoDataType] WHERE [propertyEditorAlias] = 'Umbraco.NestedContent') AND ISJSON(CAST(textValue as nvarchar(max))) = 1

OPEN @MyCursor
FETCH NEXT FROM @MyCursor
INTO @ItemID, @ItemTextValue

WHILE @@FETCH_STATUS = 0
    BEGIN

        DECLARE @outerJson NVARCHAR(MAX);
        DECLARE @innerJson NVARCHAR(MAX);

        SET @outerJson = @ItemTextValue

        DECLARE @Outer_Counter int
        DECLARE @Outer_Count int

        DECLARE @Inner_Counter int
        DECLARE @Inner_Count int

        SET @Outer_Counter = 0;
        SET @Outer_Count = (SELECT COUNT(*) FROM OPENJSON(@outerJson))

        WHILE @Outer_Counter < @Outer_Count
            BEGIN

                SET @outerJson = JSON_MODIFY(@outerJson,'$[' + cast(@Outer_Counter as nvarchar(10)) + '].key', '' + cast(newid() as nvarchar(60)) + '')

                SET @Inner_Counter = 0;
                SET @Inner_Count = (SELECT COUNT(*) FROM OPENJSON(@outerJson,'$[' + cast(@Outer_Counter as nvarchar(10)) + '].items'))

                PRINT 'Item Id:' + CAST(@ItemID as nvarchar(10)) + ' Item Count: ' + CAST(@Inner_Count as nvarchar(10))

                WHILE @Inner_Counter < @Inner_Count
                    BEGIN
                        IF (@Inner_Counter > 0)
                            BEGIN
                                SET @outerJson = @innerJson
                            END

                        SET @innerJson = JSON_MODIFY(@outerJson,'$[' + cast(@Outer_Counter as nvarchar(10)) + '].items[' + cast(@Inner_Counter as nvarchar(10)) + '].key', '' + cast(newid() as nvarchar(60)) + '')         

                        PRINT 'Items: ' + CAST(@innerJson as nvarchar(max))

                        SET @Inner_Counter = @Inner_Counter + 1; 
                    END

                    SET @Outer_Counter = @Outer_Counter + 1;

            END;

        IF (@innerJson IS NULL OR LEN(RTRIM(LTRIM(@innerJson))) = 0)
            BEGIN TRY
                UPDATE umbracoPropertyData SET textValue = @outerJson WHERE id = @ItemID
            END TRY
            BEGIN CATCH
                PRINT 'Outer Error: ' +  CAST(@ItemID as nvarchar(max)) + char(13) + char(10)
            END CATCH
        ELSE
            BEGIN TRY
                UPDATE umbracoPropertyData SET textValue = @innerJson WHERE id = @ItemID
            END TRY
            BEGIN CATCH
                PRINT 'Inner Error: ' + CAST(@ItemID as nvarchar(max))
            END CATCH

            SET @innerJson = ''

FETCH NEXT FROM @MyCursor
INTO @ItemID, @ItemTextValue

END

CLOSE @MyCursor; DEALLOCATE @MyCursor;

END;