I would be very happy if the Venture became closer to the Mule in role, because I use Mule's in my fleets all the way through the midgame. The biggest requirement I see is to bump the base burn to 8 and remove the need for militarized subsystems entirely - that brings the crew requirement in line and frees up 15 OP.
For cargo, the Mule is 250 to the Tarsus' 350 and Buffalo's 400, but thats ok because of its combat abilities. If the Venture is going to fill the 'take it for the logistics, deploy it because its tough enough' role, it needs to compete with the Colossus. 600 would give Venture:Colossus roughly the same ratio as Mule:Buffalo so I think that works.
Differentiating between the Apogee and Venture in combat: Both are slow, but the Apogee is 20 speed faster. Both are tanks, but of different types. The Apogee has a far better weapons package, though its mount placement is awkward, and a lot more flux and OP. To me this seems like the difference between a warship and a carrier, which sort of makes sense as the Venture has its mining pods. Playing up this difference could be the way to go: unlocking the fighter bay to make it free, and maybe even add a second bay. Otoh, that starts to step on the Mora's toes, as it is a slow brick with fighter bays. So crazy idea: give the Venture another 2 mining pod wings for a total of 3. That goes from 'some meat shields and lasers' to 'actually decent PD and meatshield grid'.
Out of combat, having the Venture have lots of cargo but relatively low fuel is moderately different from the Apogee's medium cargo and high fuel + exploration. It makes the Venture better for in core trading purposes and raiding, and the Apogee better for exploration.