LineageOS is the most popular custom ROM around, not only because it brings a stock-like experience to phones and tablets, but also because it has kept many devices supported long after the manufacturer dropped support.
https://www.androidpolice.com/2021/02/16/lineageos-ends-android-9-pie-support-cutting-off-24-phones-and-tablets/
https://www.androidpolice.com/2021/02/16/lineageos-ends-android-9-pie-support-cutting-off-24-phones-and-tablets/