Playdate screen (Sharp Memory LCD) is actually quite like eink: rows that aren't updated retain their display and use almost no power.
Playdate SDK Sprite system allows you to do partial redraw/refresh. Of course it only works with some game designs. It won't work for your rain.
I recommend taking a look at the SDK Examples, make sure to turn on "Highlight Screen Updates" in the Simulator, and immerse yourself in the docs to learn more about this.
Here's a recent thread where implementations are discussed:
https://devforum.play.date/t/frame-independent-games-brainstorm/16628