> The downside is that Apple Mail isn’t designed to provide the same level of customization as Thunderbird.

No shit. I spent a week vibe coding swift to set Reply-To conditionally. At the end? I find I can set X-Anything but the canonical critical list of "not these" simply can't be set and what's on that list is undocumented.

Yet, you can set reply-to in gui, and you can applescript setting it as gui actions.