Earlier quoted context omitted.
> This is difficult and time consuming so start now. A specific difficulty you are likely to face: Your expert employees are busy doing their jobs. Writing requirements means taking attention away from that work - a difficult proposition, especially if they have managers who will ask why their KPIs are slipping.
Maybe a nit pick, but I would not accept the expert employees writing requirement documents that are "thrown over the wall" to the dev team. You need personal access to talk to the experts anytime some question comes up. This means 1-2 experts work part time as PMs more or less, which will impact their regular work. Costly, but I don't trust any other way.
What you need is someone with significant dev experience, but enough outside exposure to step into the PM role. Ideally, it'd be someone with interest in user experience research, or a personality that connect outside of a dev team. They should be able to perform this role in a mechanical fashion to avoid a dedicated PM.
Doing this will be effective enough until things are at the point where a dedicated PM can be justified. If there is a struggle with the user experience aspects and getting knowledge out of the experts, you can get expertise from an agency on a short-term basis.
Adding a pure PM is going to significantly slow you down unless are one of the rare switchers who flip between dev, PM, and dev manager roles. People don't know what to make of these people, but they exist, especially in the freelance contract world. People will only full-time experience tend to look down on them because they can't understand it.