I think probably the reason Post-Crisis and Pre-Crisis don't mix as well as people would hope is because there are at least some ideological differences between the eras and the fans of those eras. You see the same discussion that surround particular topics and they all are usually focused on identity, heritage, or power. Human or Alien, dead parents or living parents, Clark or Superman, Outsider or well adjusted, whats the nature of his relationship with Batman, marriage or no, hows his work life, what was his childhood, etc. You don't see too much on which version of Metallo should be in play, Lex seems to have naturally settled into a mix of mad scientist and businessman, no one really cares one way or another about Keith White, or whats going on with Jim Harper these days. Many of things in the second list tend to just sort themselves out because they either change to fit the times or are forgotten.
The question of Pre-Crisis vs Post-Crisis is a question of the person that is called Superman. Who is he? For Post-Crisis I think the idea is that they shifted him as close to the general agreed upon view of "normal" as possible; normal childhood, powers held back for as long as possible, normal work life, human heritage is preeminent in his life, normal wife, etc. For Pre-Crisis Superman normal is relative. Post-Crisis was probably more relateable but Pre-Crisis feels more unique and in this day and age probably a lot more relevant. Then you've got the powe rlevel questions which again Post-Crisis shoots lower for the hope of relateability while Pre-Crisis didn't really bother with limits and his power just went up and up and up. Again Post-Crisis probably is more relateable, but Pre-Crisis feels more relevant given the Rusos were just bragging about Captain Marvel being the most powerful Avenger and whatnot. There are just plain ideological differences that are hard to rectify with each other; people say the Kent, Lois, and having a regular 9-5 keeps him grounded and interested in protecting the Earth but that doesn't align with Pre-Crisis Superman sensibilities who told Kara in one story that he fought crime on Earth because he loved it.
Pre-Crisis Superman and Post-Crisis Superman are kind of different in the way Barry Allen and Wally West or the Green Lanterns are different and I think are fanbase is closer to theirs than it is to Wonder Woman or Batman who also went through COIE but are more or less seen as one whole unit though time rather than several different characters that share lore and names.
The stuff that associated with each era imo is less important because that stuff just sorts itself out. Initially Krypto got the boot after COIE because they thought a flying dog was silly but after a while they declinched and realized it wasn't anymore silly than a flying man. Like wise Superman's penchant for sprouting new powers at will never returned after they were deemphasized in the "Bronze Age" because the powerset he already had was massive and pretty iconic. Stuff tends to work itself out and from what I've seen both sides acclimate pretty quickly to stuff being changed anyways it's the man at the center thats the problem.