Help needed with creating SelectRows and Group function

Thank you for the reply!

I only want to take into account past days and not today so Date.IsInPreviousNDays should serve my purpose.

I've been able to refine my function as follows:

let
  Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WKk4tLs7Mz4svLkksKlHSgfNLi1OLgFyXxMycyvji0lylWJ1oJUt9IwMgMjIGygSkFhXn5ykYApkmMEkjLJKmMElTLJIWMEljNEkjINMYl04jZGPNsEjCHWSORdJcKTYWAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t, Column3 = _t]),
  #"Promoted headers" = 
Table.PromoteHeaders(Source, [PromoteAllScalars = true]),
  #"Changed column type" = Table.TransformColumnTypes(#"Promoted headers", {{"session_start", type date}, {"session_user", type text}, {"Daily_sum", Int64.Type}}),

    CustomFunction = (n as number, user as text, initialN as number) as table =>
    let
      FilteredRows = Table.SelectRows(#"Changed column type", each [session_user] = user and Date.IsInPreviousNDays([session_start], n)),
      GroupedRows = Table.Group(FilteredRows, {"session_user"}, {{"Sum", each List.Sum([Daily_sum]), type nullable number}}),
      Result = if Table.IsEmpty(GroupedRows) or List.Min(GroupedRows[Sum]) < 10 then Table.AddColumn(Table.AddColumn(GroupedRows, "Final N Value", each n), "First Try", each n = initialN) else @CustomFunction(n-1, user, initialN)
    in
      Result,

  Users = List.Distinct(#"Changed column type"[session_user]),
  DaysToFetch = 60,
  FinalResult = Table.FromRecords(List.Combine(List.Transform(Users, each Table.ToRecords(CustomFunction(DaysToFetch, _, DaysToFetch))))),
  Custom = Table.AddColumn(FinalResult, "Custom", each Date.AddDays( Date.From(DateTime.FixedLocalNow()),-[Final N Value]), type date),
  #"Custom 1" = 
Table.AddColumn(#"Custom", "not_valid_anymore", each Date.AddDays( Date.From(DateTime.FixedLocalNow()), if [First Try] = true then 0 else DaysToFetch - [Final N Value]), type date)
in
  #"Custom 1"

This pretty much gets me what I need.