Tuvix would have been completely safe if he just killed the Doctor.
Actually?
It's only 6 episodes until the Doctor dies of old age in The Swarm.
It's a mercy killing
https://getyarn.io/yarn-clip/29825ae7-d441-4691-a100-99cad700a5bb
As far as the no where to run thing goes...
In 1 episode he would have been Captain after Janeway and Tuvok get nasty insect bites.
In 2 episodes, he could have signed up with Seska, or been trapped on an L class planet where they didn't have the technology to split him any longer.
In 10 episodes, he could have bilked off to 20th century Earth. Fashioned a subspace transpoder from ordinary household appliances, and hitchhiked off world and out of histories way.
In 11 episodes Kes stopped caring about Neelix and welcomed the kitchen rats departure from her life.
In 12 episodes, the Q could have split Tuvix, or made the rest of the crew forget that there had ever been a Neelix and Tuvok, depending on who sucked up harder.
In 14 episodes Tuvix became a convicted drug dealer, and killed a cop. Janeway should have kicked Neelix off the ship at this point. Drugs are bad.
In 15 episodes, a hot girl, who turns out to be a cronenberg in a box, invites Tuvok to move in with her and make a whole lot of whoopee.
You get the idea.
Is Tuvixing superior to Assimilation? Would the Borg upskill their tool for networking minds if someone told them that this was a possibility, which is why the Borg could never see Tuvix a valid life choice. Otherwise the universe would end soon after trillions of trillions of individuals had smashed together into one perfect snobby elite prick.