This project was intended to be a 2-3 person co-authorship, approached professionally, but enjoyably.
The aim was to collaboratively conduct market research, worldbuilding, outling, drafting, etc., so that every participant contributes ideas, shapes the project and becomes invested. So I...
Writing the first entry of the series in a way that it can hold up as a 'standalone' book is probably a good approach. The publishers seem to care more about marketability (than book quality) though, which equates to social media following/existing network.