You can change this, but it requires you to change the code in the postgresql driver sourcecode (it's available to you in the sourcecode archive).
In PostgreSqlDBDriver.cs, there are two methods, FillNETToDBTypeConversionsList and FillDBTypeSortOrderList. The first one defines all model first mapping filters. These are used by the designer to find a DB type for a .NET type. You see there are 5 DB types mapped to DateTime, as that's the situation with npgsql: it maps 5 types to DateTime.
In FillDBTypeSortOrderList, the sort order is specified, which type is taken first. We defined the 'most common usage' types first, which is Date:
this.SortOrderPerDBType.Add((int)PgSqlDbTypes.Date, 0);
this.SortOrderPerDBType.Add((int)PgSqlDbTypes.Timestamp, 1);
this.SortOrderPerDBType.Add((int)PgSqlDbTypes.Timestamptz, 2);
this.SortOrderPerDBType.Add((int)PgSqlDbTypes.Time, 3);
this.SortOrderPerDBType.Add((int)PgSqlDbTypes.Timetz, 4);
If you change it into:
this.SortOrderPerDBType.Add((int)PgSqlDbTypes.Timestamptz, 0);
this.SortOrderPerDBType.Add((int)PgSqlDbTypes.Date, 1);
this.SortOrderPerDBType.Add((int)PgSqlDbTypes.Timestamp, 2);
this.SortOrderPerDBType.Add((int)PgSqlDbTypes.Time, 3);
this.SortOrderPerDBType.Add((int)PgSqlDbTypes.Timetz, 4);
it will pick Timestamptz always.
We didn't implement a UI system to configure this manually in the designer, as most of the time you don't need this as model first is basically about 'pick a DB type for me' and forget it.
Another workaround could be this: you work with Date in the model, you export DDL SQL script from the designer. Open the .sql file in a text editor and change the Date types to Timestamptz. Then run the script on the database server. Afterwards, refresh the catalog in the designer, which will change all Date types to Timestamptz. The model types stay the same as both are mapped to DateTime.