What’s the most productive way to study for the SalesForce platform foundations exam? I’ve already trained and logged and completed the recommended 34 hours of modules, reading, quizzes, etc. The certification prep seems to be very limited

This module is really good: 

https://trailhead.salesforce.com/content/learn/modules/cert-prep-salesforce-certified-associate

Also, Focus on Force practice exams are the best: 

https://focusonforce.com/salesforce-platform-foundations-certification-guide/

I really like this video course: 

https://salesforceupskill.com/p/prepare-for-the-salesforce-certified-associate-exam