Most versions of the mod allow you to bind a hotkey (usually 'G' or 'B') to turn the effect on and off instantly.
Lighting Up the Dark: A Deep Dive into the Fullbright 1.12.2 Mod fullbright 1.12.2 mod
It is not considered a "cheat" on most private modpacks (like SkyFactory 4 or All the Mods 3 ), but public servers often ban it. Most versions of the mod allow you to