If going for any custom skin based phone at the price point mentioned, do expect lags and performance issues.
M21 is a relatively good phone for the use case mentioned. However, it will get slow with time and you need to regularly clean your phone.
Other option would be to go for Nokia phones. Yes, the hardware might be dated compared to the competition, but it shouldn't be an issue in your case. Stock Android will anyday offer smooth experience over custom-skin rom.
G20, 5.4 can be a good options. They are a bit pricier but is worth it.