I don’t see how bounces will be handled in this scenario. Bounces go to the envelope from, not the address in the Reply-To. So if your envelope from isn’t capable of handling VERP-tagged email, you’ll still be in the soup.
In the topic you reference, you don’t explain why DKIM signing against the domain in the From: header doesn’t work. That should provide DMARC alignment, even if SPF fails. That is the approach I would take, if I were in your position (either that, or take to the admins of the yyy.com incoming SMTP server with a pitchfork; what kind of MTA doesn’t support any kind of detail addressing?!?)