You can ask for a preferred role without rejecting the flexibility that has earned the manager's trust.
Start by recognising the positive: you are happy to help the team and you know your ability to move positions is useful. Then explain that you would like to develop more depth in one position and ask whether the manager sees a realistic opportunity for a run there.
That keeps the conversation collaborative. You are not saying, 'I won't play anywhere else'; you are saying, 'Where do you think I can become most effective if I get some continuity?'
The manager may still need you to move around, especially in adult football where availability changes constantly. But even knowing your primary role can help training and preparation.
Reliability should not stop you having a football preference. The key is to make it a development conversation rather than a demand.