It's a fair point, but I'll say I'd rather stick with my current InDesign solution than get into Access and LibreOffice (if for no other reason than that I have no Windows computers and am not going to set one up for Access, and the LibreOffice UX was pretty bad last time I tried it). It's true that at its core the data is relational. In fact, the source of truth is a relational database. But it's a webapp I don't own, so I don't have direct access to the DB. I can get the data via API or CSV downloads. Going from the "real" database, through the API or export transformations, and then reconstituting it into some relational db, whether it be Access, mysql, or anything else, feels like a headache beyond what I want to deal with.
Regarding docx vs. IDML, I view this as more of a page layout problem than a word processing problem, but maybe that's just the way my brain works. I want finer control of object positioning, typography, etc. than the typical word processor is really designed to provide (even if they try). I'm very experienced with InDesign, and before that Quark and PageMaker, so it's kind of my comfort zone. Really the only reason I'm looking to change is that these days I don't do as much of that kind of work, and the InDesign cost is starting to feel like a waste of money...
Anyway, thanks for the great input, everyone. This gave me some ideas to start experimenting with and see if it's worth diving fully into.