I just don't really agree much with you hear. I also scrolled through twenty years of drafts, and also through the great RB drought of 2009-2016 (2015 exempted). I don't think this 1st-2nd round gap you suggest is really there at all, and it's not really a thing. I do think you can hit on day 3 RB's plenty (just compare it to day 3 QB's which have hit at a rate below I think 5 or 6% over the past decade), but I don't think there's any correlation beyond just the fact that there are way, way, way more opportunities to hit on RB's after say the top 30 picks because there's another 200+ picks to work with as compared to the top 30. You're going to hit more often just based on the fact that every year you have 15-20 RB's drafted between around slot 35 and Mr. Irrelevant, and from 0-3 or 4 in the top 30. Of course you have more hits.
Also worth noting is a couple of other factors:
#1: The Great RB Drought
2009-2014 was the worst era for RB prospects in the history of the NFL BY FAR. Nothing comes close, and you can add the '16 class which was also absolutely horrific.
#2: The 2017-2018 classes are generally regarded as the best back to back RB classes ever, and two of the best RB classes to come around in the past three decades in terms of depth and top end talent.
#3: Career ending injuries: Over the past 20 years a pile of RB's drafted in the top 30 or so overall have sustained career ending injuries before they even had a chance to show what they were:
Robert Edwards: Probably on his way to an all pro career considering he got a pro bowl invite as a rookie. Destroyed his knee in Hawaii in a beach football game, career over.
Fred Taylor: Nicknamed Fragile Fred and Fraud Taylor by exasperated fantasy owners during his career, he likely would have been a HOF if he hadn't suffered a series of nagging injuries, season ending injuries, and one of the most horrific descriptions of an injury I've ever heard and despite all that, he was a still logged a pile of pro bowl worthy seasons.
Michael Bennett sustained a series of injuries that destroyed his career after a promising start.
Willis McGahee had his career wrecked by an injury in his last college game.
Larry Johnson: exploded onto the scene as Priest Holmes career was derailed by injuries and then he himself lost his career to knee injuries.
Jahvid Best: Lost his career just as it was blossoming.
Ryan Mathews: Became almost a joke due to a series of injuries which derailed all but two of his 7 NFL seasons (and he produced the two lone years he was fully healthy)
CJ Spiller: Was an obvious overdraft at the time, but expected career was largely derailed by injuries which sapped his athleticism.
David Wilson: Suffered a career ending injury in his first season. Looked superb in early action before the injury.
Marcus Lattimore: Suffered career derailing injuries in college, never made it back.
All in all this would be essentially be 10 of the top 36 backs taken in round 1 since 1998+ Lattimore who was a lock to be a first rounder before his last injury in college. Nearly 1/3 of the guys drafted in that zone you're talking about either suffered career destroying injuries before they'd even really started (Edwards, Best, Wilson and Lattimore), suffered a litany of injuries that sapped their athleticism and helped to destroy their careers during their rookie contract or shortly afterwards (Bennett, Johnson, Spiller and Mathews) or played through career harming injuries that drastically reduced what could have been (Fred Taylor and Willis McGahee).
When i look at things this way, it really helps to elucidate the bust factor. There's a small cluster of guys who just flat out sucked (astericks mark guys that were scene at the time as having very high bust risk and were reaches or idiotic scouting reports): John Avery*, Ron Dayne*, Trung Candidate*, TJ Duckett, William Green, Chris Perry*, Laurence Maroney, Felix Jones, Beanie Wells and Donald Brown (I may be being a bit too harsh on Jones who disappointed but did have a career, while guys like Avery, Dayne, Candidate and Perry (hell I don't even remember Perry) just wreaked of bust, while later picks like Wells and Browns looked like reaches at the time, and proved to be reaches. All the same, these guys definitely didn't live up to billing, and it wasn't health which was to blame and there are essentially 10 of them out of 36 (if you include Lattimore in the overall total) in the past 36 years.
That's not that bad of a bust rate all things considered, that being said, if you add in the guys who had career killing injuries early, it starts to look worse, but career killing injuries for the most part w/those guys weren't linked to predictable factors (those guys weren't largely BMI risks).
To me, the draft looks like it has 3 core tiers of RB's plus a bunch of satellite backs:
I think you rank them as you see them, and take the ones that represent the best value and fit for your team/or superlative talent and you figure out how to use it rather than force it:
For me that's:
Then a big Tier 3 that I wouldn't use a top 2 round pick on. There are guys in that third group that have interesting markers but also a lot of concerns: Walton (is he still hurt hence the god awful combine), Kelly (why was his combine so awful), Roc Thomas (Former five star recruit and small school transfer, is he better than we think?), Ronald Jones (a bit too small for what we want to do), Kerryon Johnson (agility score was bad, bp was horrible, and he didnt run the 40 at the combine, all red flags), Kalen Ballage (production was god awful considering athleticsm, weak bp backs up argument that he is another michael westbunk (not a lot of passion for the game), Scarbrough (old and overage and injury prone but crazy athletic still), and Justin Jackson (much more athletic at combine then expected, but a ton of wear on his tires at Northwestern).
There are guys in that third tier that I find interesting and would take a flyer on, but I think you take one of them rather than one of the guys in my top 6, odds are you're going to get a bust to add to a bench already full of them. I would look at the big 6, and grab the one that makes the most sense, and if you want to take another swing, see if Walton and Kelly have better pro days, and/or you can take swings at guys like Thomas, Johnson, Scarbrough and Jackson if they fall out of the top 100, but I wouldn't take any of them in the top 3 rounds. Way too many red flags. I wouldn't touch Ballage w/o being confident in his mental make up after serious inteviews and background checks.