Every console and platform should have its own category, especially older ones.
If you look at Retro right now, you'll find over 20 years of games represented because "Retro" is a poorly-defined term- it means something different to everyone. For many people that makes it fairly useless for discovering new interesting streamers.
Conversely, people generally know what Atari 2600, SNES, PSP, Dreamcast, etc. are, so it would be nice if streamers didn't need to resort to the punt category and getting buried in the noise (which sucks to sift through as much as it sucks to be stuck in). This would also help give better visibility to legacy computer platforms, making it easier to find the rare PC-98 streamer or people showing off cool old DOS games. Moreover, it would be helpful for people doing themed streams like "GBA Roulette" or "Trying new PC Engine games".
That's an interesting question and I don't think there are answers that satisfy everyone. It might have to be a case-by-case basis.
Like for example, I would suggest that NES/FC/FDS are all rolled into the same category because there was so much crossover in their libraries and the differences between them aren't so significant that you really GAIN a whole lot from the user's perspective by splitting them up. That might be wrong, but I think it would be annoying to have to check three different categories for "games on Nintendo's 8-bit home console"; there's no reason the decision can't be revisited in the future.
Conversely, 32x games should probably not be rolled into the Genesis/MD category (they kinda have a different flavour to them), but Sega CD games WOULD (because outside of having Red Book audio and a lot more space for data, it doesn't really change the capabilities of the system). Likewise for PC Engine/TurboGrafx and their CD libraries. And, to some extent, that was the last generation where it actually mattered, so there's not TOO many edge cases to consider anyway.
Rigour and attention to accuracy is only good until it starts getting in the way of users finding things they want. There are ways around this (for example, the NES/FC category might actually CONTAIN the FDS category, so one could drill down if they have an interest in doing so while still making sure that the JP version of Zelda is still represented alongside its overseas version), but I think anything is better than what we have now.
I agree that every platform has its own category. The same game may have different content from another perspective depending on the platform used for porting and remaking. Clarifying the platform will be a big deal for fans of the game.
I basically agree with your proposal, but I have one question. For example, how to deal with the differences between FC and NES. They basically mean the same thing, but I think it needs to be discussed whether they can be treated as the same category.