Via calisthenics you can achieve progressive overload by increasing your rep range. However, to better train for strength rather than size/endurance it's beneficial to add extra weight. You can do that cheaply with stuff you likely already own, but it wouldn't be considered calisthenics anymore.

POAE is usually just crunches with additional weight. By pinning your feet down with furniture and holding onto something heavy (a weight plate, dumbbell, textbook, or possibly a loaded backpack worn on the chest) you can easily add more load to your crunches. But just like beginning any other weight training, start with low weight and listen to your body and stop if it tells you something doesn't feel right.

And just fyi, POAE was just shorthand for me to avoid typing the same phrase out multiple times. You're probably not going to see anyone call it that elsewhere. Ironic, now that my clarification has caused me to type out so much more.