Relative difficulty of E-W route between Glacier Pass and upper Lyell Fork?

I’m planning a 3-person trip with my brother and dad for next fall. My dad will be 60 and in average shape for someone who doesn’t backpack often. Our last long trip was 2 years ago, and he was very tired at the end of a ~10-mile day with 3500-ft gained/lost, all at ~9k elevation, carrying a full pack (~35-lbs skin out), at the beginning of a 9-day outing.

We want to get to the far upper Lyell Fork drainage (caltopo link below) in early to mid September next year. In short, what’s the easiest route out there? It’ll be at least 2 days in for us no matter what, but I’m trying to balance elevation and distance. I’m not worried about the rest of the trip. We’ll either hike down into the valley or reverse our route.

Routes on map:  https://caltopo.com/m/4A1V

• RED: Start at Agnew Mdws:  ~25 miles, +12k/-9k elev chg
• BLUE: Start at Lyell Canyon:  ~35 miles, +13k/-10k elev chg
• PURPLE: Start at Granite Crk:  ~25 mi, +7.5k/-3.5k elev chg

Of the mapped options, seem like Granite Creek route is least difficult? It also has the benefit of being a section of trail none of us have done before, and probably has the least scrambling?

My dad also floated the idea of having himself brought in on a pack horse to skip a long day 1, and meeting my brother and I. But I don’t think my dad actually wants to do that. So I want to plan something that we all can enjoy together.

What would you choose? Do different? Any thoughts are greatly appreciated. I’m in the brainstorm phase.