Maybe we both "just" need to find a niche (or simple enough) product that can be started as an extended hobby. But I fear these are often not very interesting or won't even make us millionaires. How about electric tooth brushes, have these any relevant flaws these days?

Another small thing: We have a cat toy that moves via vibration (they love it), but the 3 tiny 312-type cells only survive for at most 30m. We even tried those specced for hearing aids (180mAh instead of 30mAh), but they seem to have trouble with the current and don't live much longer; so using it is not only moderately expansive, but produces a lot of unnecessary waste. A small LiPo with USB-C charger could fit, but of course then the margin would drop sharply.