Route advice

I am planning to use around four of my six weeks of school holidays in summer to go on a bike trip. My main question is, if there are any places i should skip / go around on the route shown in the image or if there are any major things more or less close by that i should visit?

Some more details:
I’m seventeen years old and i would turn 18 during the trip, i have good friend who will accompany me probably the whole way and maybe a third person who would join us until Budapest – he would already be above 18 years old. Could there be any issues with being a minor in these foreign countries?
Route wise i will start from my hometown in Würzburg Germany and cycle until prague, although i’m contemplating to ride a train closer to the czech border skipping the first one or two days in the area around here since i know it pretty well and i’m excited to see new landscapes, towns and other stuff. From prague the tour goes to vienna, bratislava continuing on the slovakian side of the border until budapest. From there it goes along the balaton lake until yet another capital, zagreb in croatia. Then i’m planning to go through croatia until the adriatic sea to the last major town: triest in italy. For the last stretch i would go up the foothills of the alps back to villach in austria from where there are good railway connections to my hometown.
For the distance and elevation i’m quite confident even though i’ve done nothing comparable yet. Still i wouldn’t say i have no experience at all: i’ve done a six day cycling trip last year in bavaria, multiple one-day tours and relatively recent a two day tour in the winter and am planning to do a few more of them before this summer. I’m familiar with all the borrowed camping gear from my dad and know how to fix the most important things on my bicycle with available tools of course. I’m going to the gym regularly, started running this school year and think around 100km per day should be conceivable depending on if there is a major town we visit this day or anyone getting sick.

If you think there are any other problems we could face but solve in advance, i would appreciate the help.