First of all, for “offline” rendering it would be best to set a noise threshold in the Render side panel (e.g. 0.01) which will make later sampling passes much faster as many pixels wouldn’t need sampling. This should allow you to set a higher sample limit (e.g. thousands) for the same amount of render time.
The flickering is most likely caused by the checkerboarded real-time denoising in Lavina. If you don’t have refractive objects in the scene (glass) you could disable refractions and this flickering should go away, unless it’s caused by something else. However it seems you do have glass near the stairs. In this case you could try simply disabling the denoiser (slider at 0) and using thousands of samples per frame together with a noise threshold. There may be some fireflies in this case though.
If you could send us the scene, it will be very helpful.
There is a always the possibility that a bug in the shading code is causing fireflies.
Or maybe you have lots of lights, which can also increase the general noise level, since with the light tree we can sample only one of those lights per AA sample.