you may leave the work and duration in the task empty and enter work in the assignment rows.
I had missed that. That's great, thanks! A possible improvement might be to show the sum of the given work along the task between parentheses to denote that it is a derived value. Idea:

For percentages, it would be, say, (50%), etc.
Does this help you further?
Thanks for pointing out that I can throw the project file to an AI and ask questions about it. Yes, that's useful, although, if I may insist, it would be nice to have a visual indication for underallocated tasks. That might be as simple as as having a different color, shape or style for the symbol on the left on the task (e.g., a triangle with a dashed border).