Can I work on multiple trim sizes as separate projects?

Yes. Each trim size has its own render history and can be developed as a separate project. You can keep a Kindle edition, one or more paperback sizes, and custom printer sizes for the same cover without replacing the other versions.

Create and render the front for each required trim before building its matching print cover. The custom inch and millimeter fields can also be used for printers other than KDP when you know their exact page dimensions and bleed requirements.