Well, let me see if I can answer everything 
First of all: Yes, the filters I use have been coming together through extensive trial and error (including the order in which they are applied, for some of them are recommended to be used before another or even can’t function after another - two examples: The Derainbowing filter “DeCross” has to be applied before QTGMC since it won’t work after your source has been deinterlaced; another example: It is better to apply color banding filters close to the end of your script, since other filters can sometimes alter chrominance and luminance, which may influence the effectiveness of color banding filters).
If your machine can’t handle x265 placebo, you can always switch to a codec that is less “work-intense”: x264 offers a few options there, the preset also helps when being adjusted (sometimes the differences are marginal truly, I just wanted to start with the best that was possible on my end).
Lagarith is a lossless codec if I remember correctly, so you would probably have to watch out for the file size, also, back when I considered using it, not every software could handle lossless/uncompressed input, so that was a reason for me to stay away.
ProRes does have some advantages, but I found that some older sources reacted kind of funny in the realm of color representation when being rendered with ProRes, so I guess that would depend on the kind of color matrix or color transfer and how it was handled.
Can’t speak to FFV1, never tried it, sorry.
Adding filters to StaxRip is “relatively” easy: Find the filter (or in this case the dll.-file) online, download it and copy it into the AviSynth directory of the program - it is also possible to do custom scripts, but I have little experience there, I mainly tweaked settings on filters that were already there. Generally speaking, StaxRip will “tell you” if a function is possible or not (usually you will get a red notice at the bottom, something like “Don’t know what Funtion x means” or words to that effect).
As for general advice on filters for issues present in your source video, I can only speak from my personal experience with older DVDs in PAL standard, so these may not apply to NTSC sources - however, I sometimes do work with NTSC, in some cases these filters may work here as well, although you may have to adjust the settings a bit.
Let’s have it short and sweet:
- Issue: Rainbowing/Cross Color Effects - Filter: “DeCross”
(I like how this filter works on static as well as moving areas and leaves the luminance alone)
- Issue: (De-)Interlacing or Handling Hybrid Sources of Progressive and Interlaced - Filter: “QTGMC”
(One of my favorite and most powerful tools: Basically not just “a” filter, but rather a whole bunch of functions which include Deinterlacing capabilities, repair for progressive input, sharpening, source matching, noise filtering, grain handling, FPS adjustments and interpolation and much more)
- Issue: Block/Compression Artefacts (often found on DVDs) - Filter: “DeBlock QED”
(Low Bitrates and Disc Storage limitations often resulted in poorly handled compression which caused block artefacting, this filters lets you set precise values on how strong the Deblocking happens, also you have the opportunity to specify how block edges and block internals are handled, which is very nice, since often the edges are more noticeable than what happens inside the block, so unlike other deblockers, with this one you can influence how much you need to “flatten” the edges without losing internal detail)
- Issue: Haloing - Filter: “Fine DeHalo”
(Takes edges into acount and provides contra-sharpening, so you don’t sacrifice important lines, which also happens to be very important if you are working on anime or cartoon sources, since the thickness of the lines there can sometimes be confused for Halos, this filter can handle them pretty well)
- Issue: Ringing - Filter “HQDeringmod”
(Smart smoother that only smoothens out areas around egdes, prevents detail loss)
- Issue: Aliasing - Filter “MAA2”
(let’s you set specific values on how to handle luminance, chrominance or both - very useful)
- Issue: Softness/Out of Focus source - Filters: “pSharpen” and “LSFmod”
(pSharpen is pretty straight forward and avoids oversharpening well; LSFmod is a bit more complex but offers a lot more options, when in doubt, use pSharpen)
- Issue: Color Banding - Filter “GradFun3”
(uses dithering to handle banding and works well with sources that were encoded “lossy”)
Hope that helps, but at the end of the day, just speaking from my experiences, there is no “universal answer”.
