I had that original Marvel Spider-Man 3 Venom and I thought he was really cool, except he had a lot of paint rub on his silver, the mouth lacked teeth, and those awful ball jointed legs.
I was also not thrilled with how his body joint was and originally wanted to fix it.
I started with that lower torso to see what I could do. I cracked it open to remove the joints. I also sanded it down so that I could add one of the newer leg joints. I used the Secret Wars Cap. I also cut the thighs so the joint could work better but it is still a little rough. I also cut apart the feet joint and added a Figma ball joint. Although it looks familiar, it has a pivot now.
I tried to rework the torso joint but it was just not working as expected. To be honest, I'm rather disappointed and wished I could actually make the ball joint that the Walmart Venom has. He did get some improved movement forward and backward, but it doesn't look as good.
His mouth needed to have some teeth, so I cut what little children's teeth were there and took out the tongue. I then tried a few methods but they were not working very well. I really tip my hat to those who make those amazing symbiote mouths. So they are just some toothpicks. Luckily the tongue was able to fit back in which was nice.

Similar to the black suit Spider that I did, I hit him with black and used the same blue to be subtle. However, this plastic really took that blue a LOT more and made him more blue than black. He looks more like the smaller Venom figure that was officially released which... is fine. Then I used the same silver from the black suit webbing for this and I think it worked fine. He wasn't used in the movie and they went with a different design, which can still work in my favor should I do that movie version.