I've never had to consider the issue of sustainable travel as I'm not wealthy, and I think that's where the criticism is directed by the use of that term in contemporary social discourse. In fact, I think it's necessary to get out of the trap of covert shame motivation with terms like sustainable, which just causes cognitive dissonance (OMG have I been doing something bad??), and instead say directly what is non sustainable travel.
I'll play along since the term has already been introduced here, and treat it as what I've said I think sustainable means.
Along with the bulleted recommendations in the OP, the first and third of which are already norms for me, I would say:
Travel like you have a very limited budget, and set a very limited budget even if you have greater means. It's always best to have a financial safety cushion anyway, because something unexpected and unpleasant almost always happens in travel. It's easier to manage such surprises one has a cushion on reserve.
Don't take planes from place to place if there are multiple locations on the itinerary, take buses. Locally, don't take taxis and Ubers, take public transportation.
If it's a long trip and this is a new way of traveling for you, give yourself a break if you need it and spend a night here and there in a luxury hotel and spa, and then get back out there.
If you're really daring, limit yourself to the biggest backpack you can carry yourself, and if you still need a few more things, carry a smaller one in front. You will quickly learn organizational and time management skills you never knew you possessed, and since you won't have the space for buying a bunch of things, you'll engage more with the experience of where you are and what you're doing. You'll have memories instead of stuff, and spend more time doing something you'll remember than shopping.
