We have to do it. Some of us have to do it in different ways. Doing small things where you can is all well and good but literally none of us are making a single impact in capitalism by making different choices about how we're forced to participate in it. Focus on tearing it down together rather than picking apart individual choices because those individual choices still result in participating in capitalism.
Lentils and beans are dirt fucking cheap. So are lots of pulses, rice, peas, whole grains, etc. I understand that some of this isn't even available in some food deserts in the US, but anywhere you have a real grocery store all of this should be far cheaper than the same amount of food/protein in dairy or meat.
Honestly I think there's a bit of a cultural problem with this. I'm coming to realize that more and more people just have no idea how to cook outside Western, meat-based cuisine. It's important to not underestimate the size of that barrier for a lot of people. I eat dirt cheap but if I ever go for a meal at my parents, I get crazy expensive chicken or sausage replacements because they have no repertoire of good vegan meals, despite having all the ingredients.
Ok, so you acknowledge the existence of food deserts but are you aware of how widespread they are? They can exist even in the middle of large cities if you live in the wrong area.
Where I live I can't even BUY Lintels most of the time. the one time I found them there were $5-6 for 1/2lb. I try to eliminate as much meat as I can but a truly vegan diet would require a near doubling of my food budget. I often can't find vegetables outside the normal tomato, carrot, onion, bell pepper spread aside from broccoli and yellow squash
Also, have you considered the TIME involved in cooking things like beans and lentils? or vegan in general? Not only does it require significantly more time shopping but it takes a significant amount of time to cook vegan food to release its nutrients. a lot of people simply don't have the time to prep and cook vegan.
you might argue that everyone can make that time but no, not they can't. Nero divergent people often can barely keep themselves fed. I struggle to both to make it to the store to buy food and to cook more than shittest of food most days.
You can advocate for veganism without shaming people. Shame is the LEAST effective motivator. focus on ENABLING. the brief period of time I ate the most vegan was the few times I was the recipient of food aid that was put together by donations from local farmers. there were agricultural products in that box that don't even get sold locally.