I also think an architect would help.

You probably want to hire an architect. They can help you with space planning.

Originally, I was going to do an addition on our house, but it is just so expensive and time consuming that I am now trying to think of creative ways to better use the space I have. For example, I have a huge landing on the second floor and no linen closet so I am wondering where to put up a closet. Same issue with the master bedroom (tiny closet but large room). We have a lot of wasted space in our 100+ year-old house that I think a designer with imagination could help us figure out how to use better, but I am worried that if we just start making changes without an expert eye, we will end up making the house too cramped.

What kind of professional could help us figure out a plan to use the space better? Interior designer? Contractor? I don't anticipate spending a ton if money just to throw up some drywall to add closets, so not sure what type of person to use.

Any advice appreciated!