In this case I would implement a user role per level, and then assign that role to a user when he/she progresses. This could be done in a flow that determines the role(s) of the user when the level it is assigned. I probably would make this configurable, by linking the 'level' entity to 1 or more user roles.
The role would give the user access to the correct forms he/she needs when on that specific level.
Gerard , I think Ferry idea is great and flexible and most important configurable at runtime.
I have a question for you. Why you need to add this in the start up microflow? If I have to check whether a user reaches a level promotion or not then I would like to use sign in microflow. Any special reason for start up microflow ?