i have the solution. you need to install the “procedural vegetation editor” plugin and enable “nanite foliage” in Project settings.
I had flickering too
Way to fix it
Set Lumen to Epic preset
Set screen percentage to 100%
Use TSR anti aliasing instead of TAA
Upgrade project to UE5.7/5.8 as they have Thin TSR support which better detects thin fine geometry like leaves.
And most importantly set two sided foliage for foliage material and plug a translucency map or generate it yourself from the base color in photoshop and plug it in the subsurface input of your foliage material.
This fixes most of the shimmering and noise in my foliage, there would still be a little bit of noise if u pixel peep , to fix it completely install DLSS 4.5 and select Preset M in the DLSS Setting which is in project settings and add it your project and select DLAA. This removes all the remaining micro shimmering at 1080p resolution.