Disable Lighting

Unlike most other fullbright mods, Disable Lighting prevents your game from performing the lighting calculations entirely. This mod only runs on the client side.

If you want fullbright, you may as well make your computer not do the math for it. This has the side effect of making F3 always display Light level: 15, which is good since it's proof that it's working, but it can be bad if you still need to know how much light certain blocks have.

Note that, because the mod is so simple, you can attempt to run it on newer versions than Modrinth says it's designed for and it might work.

Source code on Github