Use Independent Voice Playback for your cellos. Notate the solo cello in a different voice (not used elsewhere) and route that voice to a solo cello patch.
Hi @konradh, since you will need, I suppose, to assign possibly different dynamics to the Solo player, keep in mind that there is a particular workflow to assign dynamics to a particular voice when you use Independent Voice Playback: you need to enter the dynamics while the caret is activated, and the caret shows the desired destination voice (with the little orange note). Then you can open the Dynamics popover (Shift+D), write your dynamics and (very important) keep Option(Alt) pressed while you confirm the popover with Enter: so the dynamic will be assigned to the chosen voice. Here an old post with a detailed video showing this workflow (in case it could be of some help):