Getting the last date based on the next

Hi FlpsJw,
Please check if the following approach works better for you. 

You can paste the query to advanced editor in PQ to see how it works

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("fc7BCoJAEAbgV1n2LOzOppHHDSU2cgWbm3jwIBnUCtmlt29GJeigp/kHPmb+upYuk5H0tshpnMujQIcXzle0FTp/EplF3jGvCuctutLLJqolsB/6QMOG9vEZ38JR1qB0oow2mhaxKieaKL1jCtuULdDZA9v4Zw2X6odnO1LIw+0euu61NEiJQrpN5xJglI7/+67pqfN+Pj51huWTMbJpvg==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"(blank)" = _t, #"(blank).1" = _t, #"(blank).2" = _t, #"(blank).3" = _t, #"(blank).4" = _t]),
    #"Promoted Headers" = Table.PromoteHeaders(Source, [PromoteAllScalars=true]),
    #"Changed Type" = Table.TransformColumnTypes(#"Promoted Headers",{{"STARTING DATE", type date}, {"TERMINATION", type date}}),
    fxLastDate = (tb) => let
addIndx = Table.AddIndexColumn(tb, "index", 0,1)
in
Table.AddColumn(addIndx, "lastDate", each if [TERMINATION] <> null then [TERMINATION] else if [index] = List.Max(addIndx[index]) then null else Date.AddDays(addIndx[STARTING DATE]{[index]+1}, -1) ),
    GroupedRows = Table.Group(#"Changed Type", {"ID"}, {{"all", each _, type table [ID=nullable text, NAME=nullable text, JOB TITLE=nullable text, STARTING DATE=nullable date, TERMINATION=nullable date]}}),
    AddedLastDate = Table.AddColumn(GroupedRows, "lastDate", each fxLastDate([all])),
    #"Removed Columns" = Table.RemoveColumns(AddedLastDate,{"all"}),
    #"Expanded lastDate" = Table.ExpandTableColumn(#"Removed Columns", "lastDate", {"NAME", "JOB TITLE", "STARTING DATE", "TERMINATION", "lastDate"}, {"NAME", "JOB TITLE", "STARTING DATE", "TERMINATION", "lastDate.1"})
in
    #"Expanded lastDate"

Output table:

You need to use Table.SelectRows inside each otherand the thrik is that you can not use each _ for both of them, so you nead to replace it by custom function definition like (x)=> 

so the whole solution is as bellow (copy and past it into advance editor)

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUfLKz8gDUo55iTmVxSUKnkC2gaG+gam+kYGRAZCjoBSrg10lWKmpvoExSKkhfqUgtYZAYy1Aak3gao2ArJCM/NzEYiDDNS89My81tQjqAkugUkNL/EohjjA00jcwQXUvLtVgN5tBDAe72RBqk5GRUmwsAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ID = _t, NAME = _t, #"JOB TITLE" = _t, #"STARTING DATE" = _t, TERMINATION = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"ID", Int64.Type}, {"NAME", type text}, {"JOB TITLE", type text}, {"STARTING DATE", type date}, {"TERMINATION", type date}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Last Day", each if [TERMINATION]<> null then [TERMINATION] else Date.AddDays(List.Min(Table.SelectRows(#"Changed Type", (x)=> x[STARTING DATE]>[STARTING DATE] and x[NAME]=_[NAME])[STARTING DATE]),-1))
in
    #"Added Custom"

it results in

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Thank you!