comparing datetime and choosing the nearest one

Given your data, the following should work:

let

//Note references to Table_1 and Table_2
//You can either put them in sepearate queries, or set them directly in this query.
    Source = Table_1,

    #"New End" = Table.ReplaceValue(
        Source,
        each [Start],
        each [ID],
        (x,y,z)as nullable datetime=>if x <> null then x
            else List.Min(List.Select(Table.SelectRows(Table_2, each [ID]=z)[End], (li)=>li>y)),
        {"End"})
in
    #"New End"