I guess they have tried to do the best they could without having the customers implement new wiring changes to bring out any
spare serial channel for the data connection so good on them for that.
I don't think these GPS units have any means of loading the flight plan via a serial link. They do have plenty of RS232 and in some cases ARINC interfaces but none of them provide a straight means of loading the route, with its waypoint names, etc.
It has been widely stated that this is to save the mfg from having to get involved in the certification of the data source (which, in the case of windoze, or even the somewhat flakey Jepp software, would be a nightmare) but I have never found a reference for this requirement.
What you could definitely do with the Garmin x30 units is to make use of the crossfill feature which is (I believe) an ARINC connection between two of them. If they both have the same database cycle, you can transmit a route from one to the other. Obviously, this interface could be faked so that any route description coming out of a PC is made to look like another Garmin x30, and away you go. I have not heard of anybody having done this, but if they did they would obviously not be advertising it. I reckon, with a protocol analyser, the specification would be a day's work and then somebody has to write a bit of code, and get an RS232 (or USB) to ARINC converter (a few hundred quid). The interconnection may even be RS232 - I have the installation manual for these but not to hand right now - which would be trivial to drive from a PC.
The higher end versions of the G1000 glass cockpits come with a keypad for waypoint etc entry - this is something pilots have been asking for for years and I don't understand why it has been so long coming.