Steve,
What I have followed of your posts reveals that you seem to be looking for an exercise that provides both cardiovascular endurance and carryover power endurance specific to GS/jerks. I don't think this exists. I think that you may need to find the mode of LSD exercise that you like, and plug away the hours. In addition, you have to find that assistance work which gets you specifically stronger in your event - jumping quarter squats, or the like ... I'm speculating. From what you tell above, it's strength/power that is lacking. But like you said, you're using an overload, so this would be expected(?)
I can tell you this, from my recent experience: after many years of short, intense work, and lots of swings, just a few months of multiple humps per week "for exercise" provided me with more density in a session like S&S. I'm not sure what your LSD program is, but Lydiard used to discuss prepping athletes for years on LSD before ramping them up to compete ... that's, "years" ... years of at least twice per week of sessions in excess of 90 minutes (one was usually into the 2-3hour range), and daily sessions of at least 30 min. Who does this today? His athletes also took gold in the short distance events - same base cycle, different peak cycle. My short-term experience seems to illustrate his ideas.
You might also consider that you are doing everything that you can to improve in your training, so just let the training effect take place ...
Matt,
Good rucks are expensive. If you're using a school book backpack, it'll fall apart as the load gets heavy.
For heavier loads, home improvement stores sell cheap bags of dry cement mix in different weights. It's dense stuff, so a lot of weight comes in a relatively small sack. Cover it thoroughly with duct tape to protect ... or, just fill a sand bag.
Don't jump right into heavier loads ... coax it up 5-10lbs per outing.
Al