Have you provided Apple your Feedback, Martin_Williams?
Posting here is no substitute for providing your Feedback directly to Apple, whether by calling Apple Support, or via the Product Feedback pages.
If you haven’t given Apple your direct Feedback, you have no justification for any complaint if they don’t “read your mind” and make the changes you desire.
Unfortunately, if one were to provide Apple the kind of Feedback we see here, nothing is likely to change, because the Feedback is less than meaningful to the Software Technicians that would need to make the changes.
May I suggest requesting to have an option to change the email authoring format (whether as a default, or on a per-email option, or both) to a plain-text (text/plain) format, rather than a rich-text format, as appears to be the current default?
This, at least, is a reasonable request, and certainly within the range of things Apple has control over.
Requesting that they make changes in order to change the behavior of some third-party software (email client) is, almost certainly, a “no go”, since Apple has no control over third-party software.
Any request that can be construed as “backtracking” on any International Internet Standards (such as email standards) is, similarly, likely to “go nowhere”.
While the email formatting change may have the desired affect upon Outlook-derived email clients, unfortunately, there is, absolutely no guarantee.
In fact, due to the misbehavior inherent in Outlook-derived email clients, any change, with the intent to cause such email clients to “do the right thing”™️, is inherently fragile: subject to being easily broken by any of a myriad tiny changes in the web of interactions involved in transferring emails.