You can download older macOS installers with the tool,
Mist.
Great find! Looks like a helpful tool. Unfortunately, it does not appear to let you download arbitrary macOS installer versions. In addition to the options available via softwareupdate --list-full-installers (on the command line), Mist does show an option for 10.15.4, but it appears to be specific to a certain MacBook Pro model and is not the correct build for general use.
Still, a helpful tool which I may use in the future. And I was able to find a few download options for earlier versions of Catalina from The Internet Archive, The Mac Go, bittorrent, etc., and I verified the contents using
these hashes. So I believe my problem is addressed for now.
I would imagine your eye strain is more likely due to your monitor screen size and resolution setting coupled with how far you are sitting away from it than any possible version of macOS.
Thank you for your input. Eye strain issues have been widely reported on recent Apple hardware and software products, both on these forums (see the link in my original post) and elsewhere. My personal issues started when I purchased a 2021 14" M1 Pro MacBook Pro, and I've spent about two years investigating the issues. While Apple has yet to publicly acknowledge the issues or provide mitigations, eye strain issues are real and seem fairly widespread (though the majority of people seem unaffected).