@sade1212:

Regarding Season(s) 5+ :
I will do a test and let you know soon, ok? But I did a sample earlier with previous settings and the results looked encouraging (but as you have said, the souce quality does get better, so the results will also look better).

Regarding the CFR at 23,976 FPS :
As I have already stated, I am working from PAL, since I have already had my experiences trying to establish a CFR from the NTSC DVDs and found it to be a horrible experience.
But still, I will explain (although I am not sure if this is applicable to NTSC sources with VFR, so please let me know what you have found out on that end).
I am using StaxRip to handle the Framerate. Here, I apply QTGMC (with a custom made and rather complicated script) for Deinterlacing in Progressive Full Repair Mode and then I use “Assume FPS” towards 23,976 FPS. Works well for PAL.

As for the rainbowing :
That’s also done with StaxRip, the filter here is called “DeCross” (which needs to be manually added as a dll.-file to the AviSynth directory and has to be applied before the Deinterlacing, otherwise it doesn’t work).
I must admit, it does not catch everything, but it reduces a lot of rainbows well enough to be “less noticeable”.

As for smearing and detail loss with faces :
That’s kind of hit and miss, depending on the source. Often, faces in the background are a real pain, since there is little detail to begin with. Still, in my case, a combination of these factors helped:

StaxRip Filters (QTGMC with adjustments on detail improvement and sharpness increase)
Using a multi-step-approach with the AI-based enhancement software “Topaz Video Enhance AI”
(First Pass: A Model called “Gaia HQ” at 100%, purpose: Deblock/Denoise & Detail Recovery
Second Pass: A Model called "Artemis LQ at 1080pHD, purpose: resolution upscale on low quality input
Third Pass: A Model called "Gaia CG at 4K, purpose: resolution upscale with Detail Enhancement)
DaVinci Resolve
(Adding Grain, Texture and Feature Recovery)

Not all the faces (especially in the background) look great, but they do look like faces with little to no distortion, which was important to me.

Also, I fairly recently started working exclusively with higher bit color spaces (10 bit in StaxRip and 16 bit in Topaz, back to 10bit in DaVinci), since the original 8 bit had banding issues which were handled quite nicely once I switched/encoded/rendered to/in/with higher bit ranges, which is important on DS9 whenever there are larger areas of the same color (like uniforms, walls or space) or sudden changes in light levels (like with flickering lights or explosions, something which almost drove me crazy during the pilot when the station lights on the promenade kept cutting in and out, causing hard lines on everything they touched).

Hope I could help, I will post two samples from my favorite season 6 episode “Sacrifice of Angels” here soon so we can compare quality between season 1 and 6 - that way we can see, if my workflow can be applied there as well.