I got a dataset of Hamburg, Germany in so called "Lagestatus 320" (LS320). The metadata describes it as "Gauss-Krueger mit ETRS89 [Lagestatus 320]". Coordinates are like 3565575.3,5936627.0, they are metric.
The epsg database does not have such a coordinate reference system. The closest option is EPSG:31467 aka "DHDN / 3-degree Gauss-Kruger zone 3". Of course, this uses the wrong ellipsoid.
If I assign that to my data, things end up ~600 meters north of their proper location.
How can I properly use this data in QGIS or any other proj-based software?