As far as I know any kind of noise/sparkling/flickering in vegetation is likely due to AA settings.
You're using AA8 which generally should give good results, so that's may be a bit surprising.
You can try a bunch of frames with AA12.
Don't simply change AA from 8 to 12 in the main render settings, because you have your sampling method customised, but I think you are aware of this.
So go to the sampling settings dialogue and double-click on "customised sampling" checkbox and you'll see that TG automatically re-determines the optimal pixel noise threshold for the new AA setting. For AA12 this is 0.025. Now again, set sampling method to 1/16th first.
The drawback of increasing the AA is that you are also using defer atmo and thus atmosphere rendering may become slower.
Atmosphere can contribute to the issues you are experiencing, so you may choose first to leave them untouched and see if the issue resolves.
Then you can disable defer atmo and increase atmosphere samples to 32 or 48 and do some tests again.
This will quite likely give better rendertimes, but may introduce noise in shadows.
On the other hand, you only use 10 atmosphere samples and in some cases you can even speed things up by increasing the number of atmosphere samples. That feels counter-intuitive, but with 1/16th first samples it might just mean that with the first AA-pass there are sufficient atmosphere samples already present! This means the noise is below the pixel noise threshold and thus no need to apply more AA to the respective pixels. This is how the adaptive AA works in TG.
So there are a couple of things to explore and try out.
Luckily, you'll develop a general sense for these settings over time, which allow you to choose "correct" settings more quickly.
Regarding your animation...
Quite nice animation
You may consider giving the camera a bit more forward/backward movement and perhaps some rotation on the Y-axis, just to make it a bit more dynamic.
What I also like to do myself, but you need something like Nuke or After Effects for it (I have AE), is to animate it at a similar camera speed like you have but then use a retime tool like Kronos to slow down the animation.
Sometimes you can even slow it down 3-4 times, which makes your animation 3-4 times longer, for "free"!