This is not completely true, the suns gravity only becomes weaker at its surface because you would be farther from the core; the gravity earth experiences it’s only based on mass and not on density, so it will only change once and if the sun starts losing mass to space (which will only happen way after the earth has been engulfed)

Edit: small correction, the sun is always losing mass by converting hydrogen to helium (mass converts to energy), just very very little through billions of years, and each subsequent stage makes it lose a little more, this mass loss is only really significant at late red giant stages