Ljubljana - Eurovelo14 - Graz

You would cross the alps (the highest peak) via car-train, so no worry needed, and lowers the elevation a fair bit.

Overall a truly beautiful route, most of it on planned bike-paths with signs.

Would recommend for bike-packing beginners as well (I certany was one), just don't forget to be prepared for all types of weather as it is very unpredictable around the alps and it might rain a bit more often than elsewhere.

Good luck to everyone doing it and feel free to alternate it to suit your needs and wishes (I did it in 6 days but would probably take more time with such weather if doing it again).

...both Graz and Ljubljana have good train connections as well as the entire route...

If you are interested in my take of this route you should check the IG account 'niksicziva'.  Of course feel free to ask if you have questions. 

Good luck and hope you find it just as beautiful as I did.