Moving a tie in a chord

[Knute Snortum]

I'm trying to adjust the Y position of a tie in a chord, but I'm
having trouble.  What works for a single tie doesn't seem to work for
a chord:

%%%
\version "2.22.2"

moveTie = {
  \once \override Tie.staff-position = -5
  \once \override Tie.direction = #DOWN
}

\relative a,, {
  \clef bass
  \time 6/8
  \moveTie a4.~ a4. |
  <\single \moveTie a~ a'~>4.( q8 <b b'> <g g'>) |
}
%%%

In the snippet, I first test whether the overrides will work on a
single tie, then try it in a chord.

I saw that there is a layout object called TieColumn, but I couldn't
figure out which property to use.  I tried a few (Y-offset, Y-extent,
extra-offset) but the tie didn't move.

Can anyone show me how to get the bottom tie of the first chord in the
second measure to move up?

--
Knute Snortum