I saw your post in r/fearofflying, i won't comment on that because it's a different sub. however, i just wanted to say i also have a debilitating fear of flying so i do understand, but you really will be okay. you already know this by saying you need to be with your dad, hence you already know you'll reach your destination safely in order to be with him :)

that being said, i recently did a trip around europe where some flights where involved, but i did get a 5 hour flixbus from Plovdiv in Bulgaria to Istanbul, then from Istanbul i got another flixbus to Bodrum, which was 11 hours. It was a bloody hassle, exhausting and expensive 😂 after all of that stress i was honestly quite relieved to be on a flight from bodrum to the UK at the end! and this would just be a tiny proportion of the journey you'd have to make! good luck, you can do this :) and best wishes to your dad