Yes, there is an 'unrestricted choice of trains' ticket (Flexpreis Europa) - but for 2nd class its about twice as expensive as the middle fare (can cancel, no choice of train). For First Class, its three times as expensive for my longest trip, Cologne to Strasbourg.
Air - routes are too short. Strasbourg to Salzburg involves a bus to FRA
@VPS - Eurail pass too expensive (I'm not gunna go 'Youth' or 'budget'

)
I'm probably over-thinking this. My timings are not urgent and not every trip is going to be a disaster.