I feel if they provided you with a way (on device) to provide a corrected version and/or mark individual transcription entirely on device, with the option to forward individual options.

E.g. Wanting to “help Siri get better” shouldn’t have to be an all-or-nothing opt-in. Though obviously individual posts are also probably less likely to be sent.

Or maybe simply sending the local model updates would be sufficient to improve things globally?

You can actually provide a corrected version to Siri by pressing the little "Tap to edit" under the transcript! I'm not sure if the correction ever get sent to Apple (maybe it's on-device training), but I've found after a while Siri accuracy improved a lot, e.g. no longer interpret "Log" as "Lock" when I said it.

Is there a way to do that for HomePod?

None that I’m aware of, sorry :(