I don't have an M1 Mac yet, nor do I use the Siri voice with VoiceOver, so I have no solution for you, but since you asked, Apple has an E-mail address accessibility@apple.com to which they reply quickly after attempting to reproduce the problems you report. Whether they actually solve them, however, is a completely different question. I sent an E-mail a year ago reporting an issue with the Samantha voice where anything over 30 characters is spelled out, and although I had a reply and managed to prove the problem using an AppleScript and even a recording of the voice, they never fixed it.

Another way to report stuff is through the Feedback Assistant app which is hidden but can be opened by typing its name in Spotlight. Feedback Assistant requires a developer license to use, but you don't have to pay anything since agreeing to the free license is enough.

Finally there's also Apple's $1 which is where the general public is expected to provide feedback; but if they aren't known to respond on the other two platforms, expect even less attention on this one.

I just tested out Siri voice on my M1 Mac and when I iteract with the speech rate stepper the value says its increasing and decreasing but the speech rate stays the same. I then tried Alex and the speech rate did change along with the value as I moved the stepper. There's definitely a bug there.