Yes, that’s correct. The idea to have the envelope-from different from the reply-to.
Since the envelope-from domain and the sending IP match, the SPF should pass but at the same time the reply can go to GMail to process responses and if the email should bounce it’ll head back to the original domain server which can then forward the bounce back to the GMail inbox as well.
It would actually look like this:
envelope-from: outgoing@yyy.com
…
From: notifications@yyy.com
To: user@contoso.com
Reply-to: my_sweet_forum+12309847801923840923502389423@gmail.com
In my setup the outgoing won’t have a VERP because my incoming SMTP doesn’t support VERP (i.e. the bounce back won’t have a VERP address), that’s why the reply-to is being sent to GMail because it supports VERP. This shouldn’t cause a DMARC failure as it does right now.