To me, the fact that the sun and all the other planets have phases, are spherical, and have satellites that orbit them is strong evidence that all large bodies tend to be spherical, and that the large body which we inhabit is very likely to be similar.
But I also grant the common FE argument that their shapes provide no bearing on earth's shape. If we want to learn about that, we have to look at the earth itself. Luckily theres plenty of evidence for the curvature of the earth right here on our planet, even though the horizon always looks flat to our eyes.