I think moving the lengthy and slow Yoda death scene / Ben force ghost scene to the start of the movie would be a horrible idea. It sits perfectly where it is, as down time between the two big set pieces.
If you’re looking for a plausible in-universe reason why it took Luke some time to rescue Han, then why not just presume that finding a Khyber crystal to power his newly built light saber took him on a quest around the galaxy. Them crystals don’t grow on trees you know!