Get all combined subDirectories with Folder.Contents() from multiple servers (mainDirectories),

Hopefully pointing to a working solution: to be further generalized by recursive call, i.e. flexible folder-levels.

Idea: Each User-Folder-Select returns parent/child folders, which is like browsing the windows explorer (standard)
(Pretty sure, there is a conformant & better standard off-the-shelf solution, but ignored or over looked here)

// fct named GetSubDirs (apology if too trivial, mimics only one step of explorer browsing)
= (MainDir as text) as table =>
    let
        // GetSubDirs queries (for each row in a table) MainDir only the next level SubDirs 
        Source = Folder.Contents(MainDir),
        SubDir = Table.SelectColumns(Table.SelectRows(Source, each ([Attributes][Directory] = true)),{"Folder Path", "Name"}) 
    in
        SubDir

calls custom-fct (probably be better & faster done as if nested)

let 
    // Sub2Dirs (table[column] with Sub2Dirs) gets for each row get all next level Sub3Dirs
    Sub2Dir = Table.TransformColumnTypes(Table.SelectColumns(Excel.CurrentWorkbook(){[Name = "Sub2Dirs"]}[Content], {"Sub2Folder"}),{{"Sub2Folder", type text}}),
    CustFct = Table.TransformColumnTypes(Table.ExpandTableColumn(Table.AddColumn(Sub2Dir, "SubDir", each GetSubDirs([Sub2Folder]), type text), "SubDir", {"Name"}, {"Sub3Dir"}),{{"Sub3Dir", type text}}),
    Result = Table.SelectColumns(Table.SelectRows(Table.AddColumn(CustFct, "Sub3Folder", each Text.Combine({[Sub2Folder], [Sub3Dir]}, ""), type text), each ([Sub3Folder] <> "")), "Sub3Folder")
in
   Result