My opinion, for all it may be worth:

Typically, during such long liturgical actions, hymns, antiphons (most of the times with psalms), and responsories are sung. Psalms without an antiphon also seem to be an acceptable option.

I would say that it might be a good idea to sing antiphons with psalms, psalms without an antiphon, or hymns during confessions. The calm and repetitive nature of psalmody and hymnody might help people to concentrate (much more than if you choose e.g. responsories or antiphons without a psalm). It is also easy to rehearse psalms or hymns that will last long enough for people to confess (again much easier than if you choose responsories).

Of course, too much repetition may bore people. To avoid that, you might think about alternating psalms and hymns in different tones.

To «stretch» the psalms, if you sing antiphons with the psalms, you may repeat the antiphon after each verse (always using the intonation after the antiphon). This is the original way of singing and is strongly suggested by several recent liturgical books.

I am sure that the Advent sections of the Liber Usualis and the Antiphonale Romanum have many suitable antiphons, psalms and hymns. What about Iuste et pie (AR 203), Rorate coeli (AR 209), Spiritus Domini (AR 211), Ad te Domine levavi (AR 213), Ecce ancilla Domini (AR 216), Ave Maria (AR 220), Consurge (AR 222), Conditor alme siderum (AR appendix 11), Vox clara ecce intonat (AR appendix 12)? This is the result of a perfunctory search in the Antiphonale...