This is a difficult one to answer because there could be a lot that is not being said, however, based on what you are saying here is some guidance.
TL;DR
Projects are temporary organisations to deliver value and they run best on collaboration and influence. The best way to work with the project team is to ask them how can I better support the project outcomes and what do you need from me?
Whilst this might seem like a capitulation, in reality, it is simply setting aside any ego and focussing on delivery. It's a leadership behaviour even if you don't lead.
Breakdown
Let's talk candidly about what is happening here.
- You are working in a PRINCE2 environment which stands for Projects in Controlled Environments.
- You feel like project plans have been created without the necessary input from you as the Subject Matter Expert
- Those plans now partially fall to you to deliver
- That is upsetting (and understandably so)
Project Management
The thing about a PRINCE2 project is that it will often have significant governance and with that, comes a lot politics and manoeuvring. It is not supposed to be like that but it often is because reporting is so crucial and so many stakeholders are involved in the governance.
It is likely that your Project Manager is under considerable pressure to produce estimates for time and/or cost which may be unreasonable.
There is only so much pushback that a Project Manager can offer to the governance and stage gate process before they will be seen as a blocker and potentially removed.
So it is easier for a PM to marginalise the scope aspect of the Project and let those details come in later.
A PM will do this because they often have leeway to change the dates and times later in a project (as much as 10% variance) so by pushing something out now, they are hoping to pass a stage gate and then deal with the consequences later.
Therefore, your key activity is to understand what pressures the Project Manager is under and then offer to support.
This will benefit you in the long run because it allows you to influence more of the project management process and ensure the scope is not demoted below time and cost.
It might feel like theatrics and the truth is, it is.
The big risk is if your Project Manager is nothing like I described and they genuinely are not respecting your role (which happens) but failing any other evidence let's give them the benefit of the doubt and assume they moved quickly not out of spite but out of pressure.
Take the PM for a coffee and ask how you can support them getting through the governance process with an accurate scope.
Hey, how are you? I know you had produce something for the project plan, I was wondering if there is any chance to influence that? I have some ideas about the scope and might better help us hit the project outcomes. Happy to help support you in the governance process as well..
Emerging Requirements
It sounds like you are beginning to explore the idea of Emerging Requirements.
Essentially, you know that not all features can be known in advance and that some have to emerge through feedback from customers and stakeholders and testing.
Your Project Manager may or may not have experience of this. In particular PRINCE2 does not have a huge overlap with emerging requirements because significant portions of the project are documented up front. Emerging requirements are much more likely to found in a project aligned with the Agile values published in the Agile Manifesto for Software Development.
That may or may not be a fruitful conversation with your PM but I suspect it will not be.
However, it is worth thinking about you reflect emerging requirements in the project, on the plan and as a set of tasks to be accomplished.
You
A lot of this will come down to ego and right now I am certain you feel like you have been disrespected and that you are angry you may have to deliver something that is not in line with your vision or expertise. You might feel like walking off and asking for another project. All of that is understandable but you can also take a different path.
The easiest way to build a scope change is to spend some time with the team to outline your idea and ask them
How might this fail?
You take them on a journey through your thinking and ask them to tear it apart with the Project Manager. Again, put your ego aside and let them come round to your way of thinking naturally.
The form of words you might use are
Is there something there we can work with? As a customer I think some of these features might have advantages or benefits.
Projects move forward in small steps, not huge evolutionary leaps so even getting some of your ideas into the plan is a victory and sets the conditions for even more collaboration.
If you think the current scope is not delivering the outcome, commit to it for a short while and then ask for a retrospective with the team to review how effective it is.
You can find guidance on retrospectives here.
I can say this honestly, in both PRINCE2 and Agile, I have seen more Subject Matter Experts be bounced from a project than I have Project Managers because they cannot balance the correct amount of firmness and collaboration. You need to sell, not tell.
I don't know how senior you are but as I have moved up in seniority in organisations I have noticed that the best leaders and team members respond to project problems with a very calm and disarming manner even when they are furious.
I sat with a Director of RS Components once who I knew played as a defender in a football team before his corporate role. It is a position which requires considerable strength and shouting. I asked how he remained calm even when projects were off track and he simply said
'I tailor my approach to the environment'.
Your ultimate aim here is to get the best possible healthcare outcome from this project but it can only be done by bringing the sponsor and the PM on board with your ideas.
Summary
- Empathise that other roles in the project are under pressure
- Offer to support and in doing so influence the scope
- Pitch your ideas to the team for critique
- Remain positive
- Influence is key
- PRINCE2 can often be a bit of manoeuvring and quite often project politics is in the background and SME's and the team have no idea
- Remain calm and friendly