I think it is always the best to support low possible OS and devices.

Amethyst for example support minimum Android 8.0 (SDK 26)
https://github.com/atrifat/amethyst/blob/3c3571eaf344d76a669b1d7903e1de5800787ffb/app/build.gradle#L14

For device testing, if possible test the app with low performance budget device such as Redmi (Xiaomi) which popular in developing countries (India, Indonesia, etc). If the app can run smoothly in low performance device then it won't be slow in high performance device (Pixel, or any Flagship phone).