Thanks for opening the floor for such an interesting topic @Frederic. You have my vote, as a rule of thumb, for anything above and beyond just steering the wheel on sand for 4 hours. For any event within your list of suggestions, consider me as the first one to sign up.
From my perspective, I would look at it beyond the sustainability lens as it will eventually become a bi-product on how we evolve our hobby with the outdoor.
I will share my 2 cents hoping to trigger some ideas on how to take things forward, under the main umbrella of your topic. Here goes:
What I think complements the hobby of "Steering the Wheel on sand for 4 hours a.k.a Dune bashing is a sense of purpose. I call it Purposeful Driving. Purposeful driving can range from anything ranging from Theme drives such as the one led by @DP1011 to clean up pink rock, to driving to remote places (Overlanding) or adventures of any sort (Driving is the just the means of transportation in this case.)
On a personal level, my objective of learning how to drive on sand was to benefit from the outdoors of the UAE. While this remains my objective to date, I have noticed many outdoor enthusiasts start with the same objective and then get stuck in Dune Bashing mode probably because it becomes their new comfort zone or they are genuinely drawn into this activity. I think based on the number of people who are stuck on Dune Bashing is the former reason (Just a personal opinion).
Trying to keep this as short as possible or this might end up to be a rant of 5 pages, the point I am trying to make is to invite drivers to think of the concept of "Purposeful Driving" and think about what was the original idea(s) that got them to learn how to drive on sand and re-focus on those ideas. By doing so, what is intended to achieve with sustainability becomes a bi-product.
Hoping that the above is taken with an open mind and by no means am I criticizing the hobby of dune bashing. I do it with Carnity as well, almost on a weekly basis, but all I am saying is that I do it to keep my driving skills sharp for the other things I like to do, that have a purpose. Once we establish purposeful driving, then that will have a good impact on the carbon footprint and different kinds of hobbies and events will emerge from it.
Just my 2 cents...