Lensing is caused by the ability of a mass to bend light, which angle can be expressed as:
$$\theta = \frac{\mu}{rc^2}$$
Where $\mu$ is the gravitational parameter, $r$ the distance to the mass, and $c$ the speed of light.
From this, we can find the closest point where light bend around the body into a focus.
$$r_{min} = \frac{r_{body}}{\sin\left(\frac{\mu}{r_{body}c^2}\right)}$$
The angle involved is usually very small, so the above can be approximated very closely as:
$$r_{min} \approx \frac{r_{body}^2 c^2}{\mu}$$
It's important to note that it's not strictly a focal point, it's a focal line extending outwards, with a minimum distance.
Using the above equation, we can show that the focal distance of the Earth is actually higher than that of the Sun, causing it to certainly not be a smaller mission.
At those distances, however, we may as well consider all significant solar system objects for lensing. Looking closely at the dimensions of the equation, we can see that the focal radius is inversely proportional to a planets density and radius. That leaves the Sun, the gas giants and Earth as promising candidates. It's not worth considering anything smaller than the Earth, since it maximises both radius and density for what remains.
- Sun: 550 AU
- Jupiter: 5,930 AU
- Neptune: 13,300 AU
- Saturn: 13,500 AU
- Earth: 15,300 AU
- Uranus: 16,600 AU
One would perhaps have to go a little farther out than 550 AU in order to block out the Sun's corona, but in conclusion, the other candidates for gravitational lensing in the solar system require going at least a magnitude further away (not to mention their comparatively much smaller lens sizes).
For designing a mission, that either means linearly increasing the transfer time, or conversely, increasing the delta-v budget. This would be problematic for a target that already has very high values for both.
