When I understand correctly own vector basemaps for Survey123 are possible only in WGS84 Web Mercator (auxilliary sphere). Is it planned that Survey123 also supports other coordinate systems (we need EPSG 2056)?
As per the documentation here, currently the Survey123 field app only supports Web Mercator Auxiliary Sphere projection.
It is possible in the future when other mapping and projection features and enhancements are supported that we will support other projections also. Currently that is not on the near term road map, but something we are aware of and will work towards.