How to reuse build steps / speed up image builder?

Hello everyone,

I need to build multiple OpenWRT images which are very similar, except for some /etc/config/ files. I was able to automate it with a simple script, but it takes a long time to run...
Since the images are very similar I was wondering if there are ways to "cache" the initial build steps and pickup from the part where the var FILES is applied.
Do you know if anything like that is possible?