How to deal with APA in-text citations in long texts?
APA style is mostly designed for defining manuscript submission rules for journal articles. 

When you seek to apply these rules to a thesis or book, you inevitably have to apply your own rules.

The general principle is that in-text citations should unambiguously identify an end of text reference. Thus, whatever principle you adopt should do this.

For a thesis, I think either approach would be fine. You could use et al across chapters or reset each chapter.

More generally, I think that if you have a combined reference list at the end of the book/thesis, then it's fine to not reset after each chapter. In contrast, if each chapter has its own reference section, then it makes sense to reset each chapter.