The multiple villain thing comes down to scriptwriting, not having anything to do with Spidey juggling multiple duties. Look how shabbily Venom, Rhino and Electro were written in their respective movies. And was there any need to tie Sandman to Uncle Ben's murder? And I'd argue that the Lizard/Connors was nothing to write home about either, despite being the solo villain in his movie.
And though Oscorp have been a fixture in all five movies, Norman was only ever prominent in the first film. I wouldn't mind him being established here in a pre-Goblin persona. A desperate and opportunistic corporate head producing supervillains for the mob bosses to distract the heroes from the real seedy dealings. Then once his "created villains" fail in the first film, he could put on the mask in the sequel.