I agree that S is the team's biggest need and if they have the opportunity to land a stud safety in the 1st, they should do it, even if it means trading up. But I think OLB is second. The team lacked the ability to generate consistent pressure without blitzing. Even with all that blitzing, they only ranked 20th in the league in sacks with 37. Cole played better late in the season, but they need a true OLB.
Upgrading at WR should be a priority, but I don't think you need to spend a 1st round pick on one. I mean, we had the 2nd best offense in the league with Riley Cooper as our #2 WR. Clearly the offense works without needing two stud WRs. And I'm not even convinced that Benjamin would be a stud.
Re: Lane Johson, you should read
Word of Muth on FootballOutsiders. He gives a pretty in-depth look at the Eagles O-line this year. Be sure to read the whole thing for more details, but here is his summary of Lane:
I think you are being unfairly harsh on Johnson. He struggled early in the season but he improved a lot as the year went on and, as the article says, looks like he could develop into a stud.