Keeping Capacity Full When Entities Have Declining Volume

That requires a bit of coding and I made it fully dynamic, so you can adjust all relevant parameters:

let

// Function, can be in a seprarate query as well
fnProductionAllocation =
(MaxMonthlyVolume as number, NumberOfEntities as number, MaxCapacity as number, MonthlyReduction as number) =>

let
    Duration = MaxCapacity/MonthlyReduction,
    ListOfPeriods = {1..Duration},
    #"Converted to Table" = Table.FromList(ListOfPeriods, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Added Index" = Table.AddIndexColumn(#"Converted to Table", "Amount", MaxCapacity, -MonthlyReduction),
    Rename = Table.RenameColumns(#"Added Index",{{"Column1", "Period"}}),
    ProductionScheme = Table.AddColumn(Rename, "SourcePeriod", each 0),
    StartingTable = #table({"Period", "Amount"}, {{1,0}}),
    ProductionPlan = List.Generate( ()=> 
    [ProdTable = StartingTable, TotalCapacity = 0, NewCapacity = 0, RemainingEntities = NumberOfEntities, Period = 0],
    each [NewCapacity]<>0 or [Period]=0,
    each [
        Period = [Period]+1,
        StartingQty = List.Sum(Table.SelectRows([ProdTable], each [Period]=Period)[Amount]),
        NewCapacity = List.Min({[RemainingEntities],Number.RoundDown((MaxMonthlyVolume - StartingQty) / MaxCapacity)}),
        TotalCapacity = StartingQty + NewCapacity*MaxCapacity,
        CapacityForecast = Table.TransformColumns(ProductionScheme, {{"Amount", each _ * NewCapacity, type number}}),
        TransformSourcePeriod = Table.TransformColumns(CapacityForecast, {{"SourcePeriod", each Period, type number}}),
        ProdTable = Table.SelectRows([ProdTable] & Table.TransformColumns(TransformSourcePeriod, {{"Period", (x)=> x + [Period], type number}}), each [Period]>=Period),
        RemainingEntities = [RemainingEntities] - NewCapacity
        ]),
    #"Converted to Table1" = Table.FromList(ProductionPlan, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Expanded Column1" = Table.ExpandRecordColumn(#"Converted to Table1", "Column1", {"ProdTable", "TotalCapacity", "NewCapacity", "RemainingEntities", "Period", "StartingQty", "CapacityForecast"}, {"ProdTable", "TotalCapacity", "NewCapacity", "RemainingEntities", "Period", "StartingQty", "CapacityForecast"})
in
    #"Expanded Column1",

// FunctionCall in a table
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45Wcs0rySypVDBU0lEyNAACIG0KJGN14FJGECGQjBlIJhYA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Entity = _t, #"Max Monthly Volume" = _t, #"Number of Entities" = _t]),
    #"Changed Type1" = Table.TransformColumnTypes(Source,{{"Number of Entities", type number}, {"Max Monthly Volume", type number}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type1", "ProductionAllocation", each fnProductionAllocation([Max Monthly Volume],[Number of Entities],100,10)),
    #"Expanded ProductionAllocation" = Table.ExpandTableColumn(#"Added Custom", "ProductionAllocation", {"ProdTable", "TotalCapacity", "NewCapacity", "RemainingEntities", "Period", "StartingQty"}, {"ProdTable", "TotalCapacity", "NewCapacity", "RemainingEntities", "Period", "StartingQty"})
in
    #"Expanded ProductionAllocation"

BTW: I think that your sample values are not correct from period 11 onwards.