Hi! There are several ways you could solve this and I think it's definitely a good idea to focus on making each sentence a selectable unit (and not highlighting each sentence token-by-token, which would be pretty inefficient).
One simple approach would be to use the choice interface with "choice_style": "multiple" and create one option per sentence, maybe grouped by paragraph or some other sensible unit. Then you can go through them and select all positive sentences, either by clicking on them, or using keyboard shortcuts.
Alternatively, a similar use case was posted on the forum a while ago and they ended up using the manual span interface, but feeding in data with a "tokens" property, but with one sentence as a "token". This would let you view the sentences in their natural flow, and you could double-click on them to select them, and even assign them different labels. See here for details: