I somehow never came across
this post here on mobileread by Tex2002ans which discusses the standardization of footnote formatting in EPUB3, and how certain devices display that information. It corroborates what I said earlier about the <aside> tag etc., though in my personal testing I also needed the footnote ID (e.g. <aside epub:type="footnote" id="note1") included for it to render properly on KOReader.
Also I forgot to mention, I'm using KOReader 2024.01 on my phone, and I believe the same version on my PocketBook Era, but whatever version is on my Era is rendering the ebooks the same way as on my phone.