In QGIS there is an entry in the tbl_datum_transform for a NTv2-grid called BWTA2017.gsb ( https://github.com/qgis/QGIS/pull/6980 ).
So theoretically it would be enough just to activate the option "Ask for datum transformation when no default is defined" and choose the grid the first time a transformation is done.
I wonder which parts of QGIS will still not make use of the chosen datum transformation that's stored in the QGIS-settings (batch-transformation? raster-transformation?).
For which use-cases is it neccessary to also update the table-entry of tbl_srs like this:
update tbl_srs set parameters='+proj=tmerc +lat_0=0 +lon_0=9 +k=1 +x_0=3500000 +y_0=0 +ellps=bessel +datum=potsdam +nadgrids=@BWTA2017.gsb,@BETA2007.gsb,null +units=m +no_defs' where srid=31467
I think the documentation of QGIS is lacking information about a crucial part of working with it.
An already asked (less specific) question concerning some generell informations about integrating NTv2-Grids in QGIS has not led to an answer from credible and/or official sources yet (even if there was a bounty offered):