Am I missing anything with my thinking on 18650 batteries?

Yes. Order some from Kumabear. You can get protected Keeppower 2600mAh cells for ~$6 each. Don't try to go cheap on batteries, it will come back to bite you everytime (eventually). It's not just about capacity, it's about safety, longevity and quality. Over time, cheap batteries don't save you anything and probably cost you more.

https://budgetlightforum.com/t/-/12159#comment-247508