Good amount of reporting can be built using Power BI- suggest to business,
But to answer:
-Items property of this vertical gallery - Filter(KPITable, ProjectID = SelectedProject.ID)
-Items property of the horizontal gallery - Filter(MonthlyValuesTable, KPIID = ThisItem.ID && Date >= DatePicker1.SelectedDate)
MonthlyValuesTable is linked to the KPITable by a foreign key