It is not necessary for Spidey to have the Sym first, in creating Venom for its solo project; unless, Webb is planning, somewhere down the line, to use the Sym as part of his franchise. Other than that, I don't see it being necessary, and only add to the delay of this particular film from being made, base on whenever Webb is ready or interested in introducing the Sym.
As far as the approach itself, Venom need to done from the POV of a villain. I have said this numerous of times. Not only that; but, Venom is a villain, and needs to be treated as such. Not as some "lethal protector"...that's not a villain; that is a Batman or Punisher character, a dark hero who flirts on the edge. Venom needs to be an all out VILLAIN; a force the Peter/Spidey has to wrestle & question himself in how far will he go, what does it take, to come up against this villain. I'm tired of this,"he's bad, but bad in a good way, protecting the innocent from unscrupulous individuals wishing to do harm....hell, that's Spidey, with the Symbiote. Don't need a "so-called" villain to be the same. So the film makers need to decide what approach they wish to take with this character.
Finally, Eddie needs to have a history, a background, to establish some foundation into who he is, and why he is so easily overcome by the Sym, and embraces it. As well, have a some kind of relationship with Peter, in one form or another, good or bad; yet, show that they are very aware or familiar with each other. The Symbiote needs a historic lore about itself as well, to demonstrate its nature & justify its existence. I have an idea how this can be accomplish within the reign of film. However, these factors, I feel are important in the creation of Venom, and justification of this character existence in the lore of villains.