Trello search and filter by due date

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.

It sounds like what you want is different notions of 'Subscribed' to a card and 'Assigned' to a card. I can't think of a good workaround for you right now (short of just noting the card owner in the description field), but we are looking at the idea of board or card subscriptions, and an individual 'card owner' might make a good plugin.