Word for a set of small side tasks that must happen before the primary tasks can continue

Solution 1:

The word preliminary might work:

coming before and usually forming a necessary prelude to something else

(source: Merriam-Webster)

Preceding or done in preparation for something fuller or more important.

(source: Lexico)

I think it captures both the necessity and the not being the main tasks.

If you need something that captures the 'in between', you might try impeding but it has less of a 'necessity' feeling.

Solution 2:

I'm going to suggest

Prerequisite

something that must exist or happen before something else can exist or happen

Prerequisite tasks are subtly implied they are not the main task, but if you need to make it clear that these tasks are not the primary or main task you could say prerequisite minor tasks or prerequisite secondary tasks.