I have been using a Ryobi power source (which according to my USB-C power meter puts out 27.3W) on a 2019 i5 MBP that came with a 61W charger. It will run normal tasks (youtube, web surfing etc) while charging the battery (albeit slowly).
An M1 MBA (which normally comes with a 30W charger) should be much more power efficient and will likely do as well.
Also, considering that my iPhone 12 is now in conservation mode (charging the battery slowly in the evening), it seems like charging slow shouldn't hurt your battery long term.