A few tricks that are helpful for me...

Lubricate the tubes and stopper before you push the tubes through the stopper holes. Any liquid will work. I use fuel. Trying to push a dry tube through a dry stopper does not work well at all. I push the tube through the stopper while the tube is still straight. That way I can press the stopper down onto the tube with the tube pressing against my worktable.

There are different alloys / hardnesses of brass tube. Some are much softer and easier to bend than others. See what your LHS has for sale and try several different kinds. The are inexpensive. I have found some that are easier to work with than the brass tubes that come in a tank kit.

I have not had much luck with aluminum tubing. It breaks easily. Maybe there are better alloys / hardnesses that would work better but I have not found any.

A Dremel cut-off wheel works well for cutting tubing. Clean up the ID with a drill bit and clean up the OD with a small file. Flush the tube out afterwards.