Trello – Can Trello breakdown a project and suggest due dates for each project

I agree with your friend about not dividing up into separate boards by type of job.

With your examples such as 'Design ideas' etc, I think they really do fit naturally within different lists on the same Board. It's exactly the sort of usage that Joel demoed in multiple videos about Trello.

The only thing I can think of, to help manage very long lists, is to use Trello's Labels system to put categories on cards, and then use filtering. You should definitely give meaningful names to the default labels.

I wrote up some tips on how to use labels recently.

Having said that, fairly early on the Trello on Trello board got huge, and eventually they reduced the scope of it. So it's does seem to be a well-known problem.

You can see their workload by selecting only their cards using the search/options tool in the right-hand sidebar. As far as their capacity, You could potentially add a board to specfically manage that metrics. AFAIK, there is no way to store that in trello; if that is an important aspect of how you are using trelly, there may exist better tools.

I suggest you might want to ask on https://workplace.stackexchange.com/ about whether you might be directly managing too many people and how you might improve efficiency in that regard.