There are various options. Easiest is a ferry from Greece to Venice, then Brenner pass to Austria, Germany, Holland, and another ferry from hook of Holland to Harwich. Venice to Munich is only 5 hours, and Munich to hook of Holland is less than 10 hours. So doable with 1 overnight stay. Alternatively you can drive all the way via Albania Croatia etc (Albania is now safe ish) it's actually quicker but further to drive through North Macedonia and Serbia because you are on a motorway all the way to Germany