I think a major part of why people lapse completely is that there exists a widespread belief that veg*nism is a hard and fast label, and falling off the wagon is irreversible. Many ex-vegans I know now eat completely omni diets that are simply at odds with their moral beliefs. I think that promoting the idea that 80/20 veganism is okay will result in a lot less animal product consumption overall. This article explains the way this black-and-white thinking negatively affects peoples' beliefs around their diet and personal identity.
Religious communities have a lot to teach us here. Picture the following recruitment strategy:
Eventually, this kind of community could function like a local atheist spiritual community of some kind, and be a platform for spreading other effective altruism ideas.
In most parts of the world, veganism is not the norm, which I believe is a big part of why people find it so challenging long-term. Focusing on universities strengthens the perception that veganism is merely part of a normal period of experimentation. Once there are people who live in communities where more than 50% of people are veg*n, I suspect it will be much easier to both retain those people and convert new people.
I use a harm-minimisation strategy when I'm in food deserts or traveling, but I suspect I'm very rare. For many vegans I know, not eating any animal products at all is part of their identity, so being forced to consume them because of a lack of availability would be incredibly emotionally stressful for them and create a lot of cognitive dissonance.