The spinoffs might not need Spidey to have a big role and they don't need him unmasked, so it's not an absolute necessity for them to use the same actor.
The new Spidey actor will be under contract with Marvel since he's appearing in Cap CW, so Sony might not even get to use him for anything Marvel's not involved with.
He will clearly be under contract with Sony as well since Sony is releasing the solo Spider-Man movie, not Disney. He can't do that picture without a multi-picture deal with Sony as well.
Sony is not going to be doing spin-offs that don't tie into the mainline Spider-Man films, they want to build a mini-universe. Having Spider-Man spin-off's that aren't connected to the actual Spider-Man films themselves is not something they'd ever do. A Venom movie that is unconnected to the Spider-Man films? A Sinister Six movie with the sinister six different than the ones that will be used in the Spider-Man movies?
That will NEVER happen. This isn't comics, Sony is not going to juggle multiple universes here.