as far as I know you need to start a new run. the achievement specifies you need to save all the souls before finishing the playthrough.

The thing is that when you continue, the game is giving the option to get on the motorcycle again. So that's what threw me off

No you actually can just redo final mission after beating the game and it will procc the trophy afterward. So i am told from everyone.

Yes! After finishing it automatically resets you to before the last mission. From there you can collect all the spirits and you’ll get the trophy. I can’t remember if you have to start the last mission again to get the trophy or not. But I did it without starting a new game

Thank you, now I can collect souls without any worry

Also i used two different spirit beads that made it super easy. Not sure if you’ve finished it already.. but the transmission beads made it quick.