I think it is always the best to support low possible OS and devices. Amethyst for example support minimum Android 8.0 (SDK 26)

GitHub

amethyst/app/build.gradle at 3c3571eaf344d76a669b1d7903e1de5800787ffb · atrifat/amethyst

Nostr client for Android. Contribute to atrifat/amethyst development by creating an account on GitHub.

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).