List.Generate, make behave like Accumulate or XL REDUCE

You can use List.Generate, but it seems cumbersome when what you want to do is accumulate results into a single record as List.Accumulate can do. List.Generate returns a List by design.

One method would be to wrap List.Generate inside List.Last

let 
  val = {12,21,34} , 
  pos = {0,1,2}, 
  head = {"a","b","c"},
 
  res =  List.Last(List.Generate(
    ()=>[idx=0, x=Record.AddField([],head{idx},val{idx})],
    each [idx] < List.Count(head),
    each [idx=[idx]+1, x=Record.AddField([x],head{idx},val{idx})],
    each Record.Combine({[x]})))
in
    res

Another method would be to return a List of Records and then combine them into a single record:

let 
  val = {12,21,34} , 
  pos = {0,1,2}, 
  head = {"a","b","c"},
 
  res = Record.Combine(List.Generate(
    ()=>[idx=0, x=Record.AddField([],head{idx},val{idx})],
    each [idx] < List.Count(head),
    each [idx=[idx]+1, x=Record.AddField([],head{idx},val{idx})],
    each [x]))
in
    res

Please note that the most efficient method (within your constraints) would be by using List.Accumulate. Theoretically, Record.Combine(List.Generate(...  should be more efficient than List.Last(List.Generate(... as one is only generating a single record for each "loop" in the former, and increasing numbers of records in the latter.