I'm not sure you need to hit a noise level lower than 5%! We do mostly animation, and with a good enough noise level and some denosing you should get acceptable results.
For me I gave up on doing any type of Cache for the solvers when we moved to Corona. All our animations are rendered with Path Tracing for both primary and secondary. We don't get any flickering in lighting, just noise. I know the tool tip says Path Tracing for interior is not the most forgiving for renders, but we usualy have objects moving around, lights animating and all kinds of things tha make it the right choice for us.
Now i'm not sure if we could reduce render time by baking the secondary solver, but my time and sanity are more important than messing with that stuff lol. The only thing we do is set our desired Pass Pimit based on a few still frames targeting to be around 5-10% % noise with about 0.75 denoising.
Some other things we do in our Max start file:
- Set corona to have a bit of highlight Clamping in the system tab, and mostly use Corona High Quality filtering.
- Lower the Max Sample Intensity in the Performance Tab.
Now I know that by doing this it will affect the physicality of the render per say, but it helps with getting rid of junk like fireflies and other artifacts, at the expense of a bit darker image that no one will ever know about :)
Take all this with a grain of salt, as we have set up Corona for how we like images, how fast we need renders and overall what works for us. But in my opinion for animations where more than just tha camera are moving, it is what works here.
You should post a video of your problem, so people can give you real feedback though.
Also
"The texture of the plaster wall has intricate details so the first thing I did was to simplify it a bit by adding a small blur on the plaster texture in Photoshop so that Corona has less work to do with the sampling. also note I use only CoronaBitmaps in this scene, with Filtering > Blur value of 1.0"This should not be happening, and you should not have to blur your map like that at all.
EDIT: Here is our work if you want to see our results.
https://iamstatic.com/work-archive/