Looks like it’s mostly been said but I’d repeat, just get the Hull ferry, it’s so much nicer than going to Calais if you’re from the North.
Rotterdam to Salzburg taking it easy is 11.5 hours every time.
Zell, Leogang etc is 1hr drive from Salzburg (always go via Lofer avoiding the autobahn)
Check out our little town (Maria Alm), pretty, nice walks, restaurants etc, 15 minutes from Leogang.
I’ll be there on and off from late August to October if you want to hook up for a ride.