What route should I take through southern France? What about after leaving Rome?

I am planning on Interrailing from Paris in may. The city I will visit for sure is Rome. After that I will head north, probably eventually to Vienna or Prague for a flight back home to Finland.

Unfortunately I have only 10 days to travel when I leave Paris and I wonder what route I should take.

Rome is quite far (16,5h average travel time by train from Paris), so I will visit some place on my way there. What would be your recommendations? Could be just a single city, maybe with an adjacent daytrip place, a village, nature, something like that. I would like the city to be small, no more than a couple hundred thousand people, as Paris and Rome are both massive. I also think Paris, Rome and (maybe) Vienna are enough art and history for me, so that isn't a focus.

I was thinking of spending a night in Strasbourg (1h45m from Paris), but it isn't really any closer to Rome, so I would spend another night in Bern (2,5-3h from Strasbourg) and try to mountain bike somewhere nearby and then head to Rome (6,5h trip).

What kind of a route should I consider if I were to go through Southern France instead? What about after leaving Rome, if I intend on reaching Vienna and/or Prague?

Only thing I know for sure is that I want to rent a mountain bike at some point and bike somewhere with mountains around or at least on big green hills.

Btw, how expected is it to miss trains or for them to be late on my trip and are there any smaller places that aren't touristy that time of year in France, Italy or Austria? I have never travelled Europe by train or bus. In theory I could also finish my journey in Rome.