I would put everything into one script add-in and have it add multiple commands for the different buttons to call (or one command with args, or a mix of the two, depending on how related different buttons are). Each command can then use functions in the same script.
Windows has ways for scripts to include other scripts, but they're a bit of a pain to use and also potentially use more memory and slow down startup vs only including the things you need in each script or combining everything into a single-script add-in:
If you have need for something that a lot of scripts might find useful, we could consider adding it to the scripting API that Opus provides.