Anyone know if would be possible to "convert" a File GDB with a set of features classes, which contain Domain values and Subtypes, in GPKG format (for editing purpose)? What I'm not understanding is, in particular, how to upload Domain values for fields in a GPKG.
As far as I know you cant import the Subtype and domain from File GDB to Geopackage.
One way to get almost the same functionality is to only import the layer to Geopackage then create a QGIS document where you set edit widget to replicate the subtype and domain (ESRI has tool to export domain as table if you export to the same GDB and import these domain table in GPKG you can use it to set up your widget ensuring that you get the exact same value in both software, if you have lot of domain it's also quicker).
This solution is only close to the ESRI way because even if you get the same editing functionality (ie. dropdown list that can be filtered depending on another field) this is only implemented at the document level and not at the database level so you have to set it up again for every new QGIS document (you can save the style to speed up creating new document)