Hello ,
Thank you for posting in the community and this is a common question with Path Tracer and you can reference theses previous discussions posts:
https://twinmotionhelp.epicgames.com/s/question/0D54z00007lFJd7CAG/flickering-in-video
https://twinmotionhelp.epicgames.com/s/question/0D54z00007uqqJqCAI/weird-artifactblurring-in-path-tracer
Keep in mind you can manually increase the sample by pixel by typing in a value and not using the slider (the max value is 65,536). Did you try with Denoiser off and fireflies off? I see you posted in the Facebook group and you will get good feedback there as well.
Here is a link to a good overview post:
https://www.facebook.com/groups/twinmotion.community/permalink/1019541698937124/
Kind regards,
Vincent B.