How to reduce temporal flickering after frame-by-frame restoration?

[Selur]

Not really,.. but here are a few thoughts about that:
You can use Filtering->Vapoursynth->Sharpen->aWarpsharp2 or Filtering->Line->Misc->Thin to thin the lines a bit, but that probably will not help much.
Alternatively maybe downscale before resizing and use something like Anime4KCPP/RealESRGAN might help.
Maybe someone over in the Discord Channel of https://openmodeldb.info/ even has a special model for filtering DB.
Downscaling by 2.51 and Upscaling using 4x_UltraSharp:

Starting from the DVDs would probably give you better results.
Alternatively using something like seedvr2 through ComfyUI might also work. (wild speculation)

Cu Selur