I was also a little concerned about how to tackle the Mash Tun at this scale. I feel like I am close to maxing our what you can realistically do in a cooler.
I recently assassinated a 15 cu chest freezer...... for a moment I thought about hanging on to it and making a big ass mash tun out of it.......
I recently brewed a batch with my 70 qt cooler with 25lbs of grain and wasn't even half full even with a pretty high water/grain ratio. I imagine I could fairly easily fit 60 lbs of grain at 1.25-1.5 qts/lb ratio in it. that would provide 30 gallons of reasonably strength beer. For a big beer you could go with a 100qt or add extract to bump your gravity up.
Or you could use only first runnings...
For what? To recirculate?
All grain is great. Takes more time, yeah, but the control over the process is worth it alone. But there are nostalgic things like the smell of the grist at mash-in and just the general process.
You could go up to partial mash for a little more control, but keeping some simplicity from extract brewing in there. But really, all grain isn't difficult, it just takes longer.