It is very shameful if iNaturalist doesn’t hire accessibility professionals to do this and relies on unpaid disabled users to do all the subtle, tiring and complex work of fixing their whole website. If they ever ask for your consultancy, I hope it is paid.
I am a new iNaturalist user and made a post about this, not only about screen readers but also talking about other concerns with accessibility, if you want to add up to it: https://forum.inaturalist.org/t/discussion-accessibility-for-inaturalist/66483
I will edit my post to add a link to your post there.