The only reason why we got these probes as far and as fast as we did was because we were able to fly by each planet and use their gravitational influence to skip to the next one, increasing velocity after each slingshot.
You are correct in that the Voyagers' launch window was unique.
Nit pick: gravity assist doesn't fundamentally rely on gravity. It extracts energy from planets' motion about the sun [1]. If we could somehow "bounce" a spacecraft off a planet's surface, it would impart momentum similar to that of a gravity assist. (In my opinion, it's a badly-named term.)
[1] https://science.nasa.gov/learn/basics-of-space-flight/primer...
When my kids were little, I showed them by holding a basketball in one hand and a tennis ball in the other, with the tennis ball on top of and touching the basketball. When I dropped them simultaneously, they fell together and bounced off the driveway — and the tennis ball took off like a rocket (no pun intended).
I'm not sure how you can make that claim. If you were to bounce off the surface, that would likely not be considered a "gravity assist", no?
It’s also a good illustration of why we call some forces strong and others weak.