As you mention the Lotti 3v mass, it seems that you are not strictly looking for something Christmassy. If the weak point in your ensemble is the bass, you might consider Monteverdi's 4v mass from "Selva morale e spirituale" because the organ (figured bass) always goes colla parte with the bass. Beware that the first edition on CPDL omits the organ part, so make sure to download the editions by Peter Rottländer. It is imitatoric throughout.

Moreover my very recent "Mass for three voices" with organ accompaniment (figured bass) might be exactly fitting for your needs (here are the score and a complete recording). The bass part is in baritone range and is mostly independent form the organ part. There are some rhythmically tricky sections, though, as we noticed when we recently sang it with our family, so make sure to plan more than one rehearsal.

If you are looking for something strictly homophonic without fugati etc., the "Eschringer Messe" for SAB & organ by Hans Werner Jost might be useful, albeit very simple. The video description of the recording on YT has links to the publisher.

In the alps, "Pastoralmessen" have been popular at Christmas (CPDL lists many examples), but most are with orchestra and they tend to be repetitive. Maybe my 3v Mass in E Major in the style of the Viennese classic is a useful alternative. It is without orchestra and only with organ accompaniment.

Delibes Messe Breve can be done with two voice parts and an organ. It's a crowd pleaser.