Rhythmically independent parts

I’m putting a song into Dorico that has in the piano, unmeasured tremolos in the right hand, and an eighth-note-based ostinato in the left hand, repeated ad lib. Later the voice enters on the third system with short rhythmic but unmeasured phrases at its own tempo. The original manuscript just has the word ostinato with repeat signs on systems after the first one so I don’t need to deal with its tempo alongside the voice’s tempo. I don’t plan on playback in Dorico.

I’m wondering about the best way to proceed. Should I create a long measure and drop the notes in where it seems appropriate? Is there a way to turn off the grid, allowing for totally free input?

My first thought (after abandoning any hope of playback) is to use a separate flow for the unmeasured phrase and to insert that flow into the main score of your music. One of the Discover Dorico YouTube videos has some info on controlling flows which might be helpful, but at the moment I do not recall which.