Let’s dispel a myth. No one can replace you in the onerous task of writing the proposal in the most technical and significantly innovative aspects of your idea. You hold the key to the innovativeness and no one else but you – however expert they may be in your field and discipline – will be able to describe your idea in a more incisive and detailed way.
But… there are parts of a proposal that are not related to Excellent science/technology/innovation, which is not the “project” itself, yet they are equally important for the quality of the overall proposal.
These parts (typically Research Data Management & Open Science, Gender Dimension, Do-Not-Significant-Harm principle, Impacts & Outcomes, Exploitation, Dissemination, and Communication) would force you to leave your “comfort zone”, and invest badly your time, to finally obtain a result that is probably not sufficiently good.
You would like an expert to take responsibility for these parts of the proposal, and you are available to contribute to the drafting of it by providing your knowledge of the sector, the subject, and the socio-economic context of reference.