The single biggest waste of time during the actual season comes from figuring out access that should have been nailed down pre-season. It's also the easiest part of physical scouting.
You can burn entire days of physical scouting just in driving around scouting for access you thought was easy or do-able from e-scouting. And as someone mentioned above, you can also uncover useful routes and 2-tracks that are just not marked on OnX. You also see landscape and vegetation layout that's impossible to perceive digitally.
On opening morning, there's no mystery at all where I'm going to be - I know exactly what my game plan is for where I'm driving, where I'm stopping, how I'm hiking, and how long each of those legs takes, to be at exactly the glassing points, food plots, and choke-points I'm going to be hitting. Including roughly at what times and roughly for how long, because I've done it before and logged that data. That is just not possible if someone hasn't nailed down access well before the season opener.
Time, energy, and mental focus are the most precious resources available during the actual season, and I don't want to burn any of that figuring out access or what I should do next. You only get a thorough and efficient game plan with pre-season scouting.
As to the challenges of a high-migration area, just scout for that. Deer spend the summers up high where it's cool, and move lower for food and warmth as the weather presses them down, obviously. You don't know exactly where the deer will be on any given week, because of weather conditions going all the way back to spring precipitation and what that means for food, so simply incorporate that ambiguity into your plan. Have high spots you'll hit first, and food/glassing/choke points down through lower altitudes you map out and know where you'll want to be if you don't see anything high.
Deer migration mapping data is also available - do a search here and you'll find at least one thread where there's a ton of resources others provided. That may help narrow down the specific glassing spots and geography you'd want to check out.