Re: "Is there a way to make Mac's spoken text sound more realistic?"
You may want to experiment with the available settings:
Change Spoken Content settings for accessibility on Mac - Apple Support
or
(Non-Apple) YouTube video discussing settings etc: https://www.youtube.com/watch?v=vBvUZBETlZU
_________________________________________________________________________________________
Re: "Is there something I'm missing, or is semi-realistic speaking still not available as part of the Mac OS?"
As well as the Mac (Spoken content > Speak selection) option, Apple provides VoiceOver on Mac in accessibility.
VoiceOver is your Mac computer’s built-in screen reader. Besides reading the screen, VoiceOver has other features, (eg: screen-navigation) that especially assist those with little or no eyesight to use their computer.
Apple provides a user-guide that may assist in making some, or full, use of VoiceOver if you want to:
VoiceOver User Guide for Mac - Apple Support
Eg: One of the articles explains how to: Learn and practice VoiceOver on Mac - Apple Support