Hi Andrew,
Of course there is nothing wrong with setting the minimum length around 4.5. In this example this setting results in the Ties ending halfway between the Noteheads, which I feel as a "loss of continuity" (for lack of better words), and this did not leave me fully satisfied. I am aware this is nitpicking, but this is why I was looking for an alternative.
My previous answer was not without problems however. First, the alignment of the Accidentals to the Notes is off, which is ugly. Second, the code lacked coherence and had hard-coded values, so it was not useful outside of this very specific case. I tinkered with it and got rid of the misaligned Accidentals by using whiteout in the Ties, and wrapped the whole thing in a more flexible shell which will be useful to me in other situations.
Attached is my updated solution, hope someone finds it useful!
tweaking-ties-and-accidentals.ly