The ability to transfer a route created using a PC and some on-line planning software to a GPS is indeed a 'missing link'. What is needed is a front panel USB stick port and 'import route' option on the a/c installation.
Plan the flight using an on-line planning tool with embedded current meteorological and NOTAM information, then download it to a USB stick. Creating lat/long waypoints by mouse-click would be a lot simpler and less error-prone than typing waypoint strings into a conventional GPS. The planning s/w should flag an alert if, for example, you plan to fly through areas of hazardous weather or through an activated danger Area. Then import the route to the a/c GPS with the USB stick - if NOTAMs have expired since the planning date/time stamp, then an alert should be given.
Irrespective of the gucciness of your spangly a/c kit, I will not be persuaded that a current chart isn't also needed as a tool of last resort - with the route pencilled in, as a minimum.
I'm a great believer in the use of GPS by PPL holders - but the user-friendliness and distracting eye candy of the more complicated all-in-one boxes leaves a lot to be desired. You'd need to fly every day to maintain sufficient familiarity with the kit in order to be able to use it safely.