Trying to create a large banner - what is the most optimal way to do this?

So, I have a very long banner that I'm trying to design. It requires a lot of photo compositing and editing, hence the use of Photoshop.

The dimensions of the print are 115cm width x 40cm height. I created a Photoshop file with those dimensions but of course it is absolutely huge and it does not let me save it. I have rasterized all layers, set the ppi to 100, increased the RAM usage of my computer and played around with other optimisation settings.

So far nothing has helped, and I still cannot save my file as a PSD. I've searched everywhere for the answer and nothing seems to be specific enough for my problem.

Ultimately when all the editing is done I want to take this file and put it in Illustrator to add text (as vectors). Is there anything I can do to make this process smoother and not completely kill my computer?

Thank you.