The 20-30* route would be my first attempted route. There is quite a difference in 20 and 30 degrees though. The 800 Cat and 800 Pro should climb 30* with two guys, tough to say about the other two sleds. I'd send the Pro and the 800Cat up solo, get to summit by any means, then on the way down lay your nice "highway". Depending on snow depth you may need to ride the sleds down this "highway" a couple times before attempting to use it to ride up with two guys. Let the snow sit and set-up a bit before climbing up the "road" with two guys.
If all four sleds can climb your "highway" with two guys, you're golden. Leave two sleds at base, two sleds go up top with all four guys. All four ski down. All four guys go up on two sleds that were at base. All four back down on sleds. Drop two sleds at base, all four guys go back up on two sleds. All four ski down. Repeat as desired. Two hill climbs per ski decent.
I'd skip the towing. Tandem is more versatile and you're not worrying about rope tension during a hill climb.
Let's say small sleds won't climb riding tandem. You can still sled lap so long as the smaller sleds will at least climb the hill solo. Ride 2 capable sleds up tandem, leaving 2 small sleds at base. All four guys ski down. Two guys ride the small sleds up, solo. Then hop on the capable sleds and ride down. All four go up, then all four down on sleds. Then tandem up on capable sleds again, leaving little sleds at base. All 4 guys ski down. Repeat. PITA as you will hill climb 3 times per ski down, but if the small sleds won't climb tandem and you don't have skins it will work. Tough to say at that point though, maybe the 5 mile lap will be more efficient.