@JonB It appears the problem was in my testing after updating the latest code here (Also it was always 10,000 data points, I wrongly wrote it as 1000.. I over-worked this issue. I will mak an edit. ). CPU usage has come down to 18% from 60%. I had done following things-

Moved LineSeries update to backend

LineSeres clear + append is replaced with replace

Qt is upgraded to 6.8.0

I am not sure which one out of this helped but results look better now. Thank you so much everyone who spent their efforts to read my long question and provide amazing ideas. Appreciate it.