My sister was a vegetarian for 15 years before I became one. She normalised it for me. I think I've normalised it for other people. This sort of ripple effect is impossible to measure but it gives us more reason to uphold admirable behaviour.

Ill respond to all thoughts

1. Correct.
2. One way of viewing altruism is that you have an energy bar of it and you should spend it wisely. The other way of viewing it is more like exercising muscles at the gym. I think in general, the 2nd view has slightly more merit and much more than we like to give it credit for since it's inconvenient. https://forum.effectivealtruism.org/posts/DBcDZJhTDgig9QNHR/altruism-sharpens-altruism
3. I don't think EA says that, at least in practice. I know of no EAs that donate all money above what they absolutely require, have no outside hobbies, etc.
4. Most of EA is inconvenient. That's fair.
5. I agree, I would also supplement with my comment I left here where similar things were discussed.

My reasons for being vegan have little to do with the direct negative effects of factory farming. They are in roughly descending order of importance.

1. A constant reminder to myself that non-human animals matter. My current day-to-day activities give nearly no reason to think about the fact that non-human animals have moral worth. This is my 2-5 times per day reminder of this fact.
2. Reduction of cognitive dissonance. It took about a year of being vegan to begin to appreciate, viscerally, that animals had moral worth. It's hard to quantify this but it is tough to think that animals have moral worth when you eat them a few times a day. This has flow-through effects on donations, cause prioritization, etc.
3. The effect it has on others. I'm not a pushy vegan at all. I hardly tell people but every now and then people notice and ask questions about it.
4. Solidarity with non-EAA animal welfare people. For better or worse, outside of EA, this seems to be a ticket to entry to be considered taking the issue seriously. I want to be able to convince them to donate to THL over a pet shelter and to SWP over dog rescue charities and the the EA AWF over Pets for Vets. They are more likely to listen to me when they see me as one of them who just happens to be doing the math.
5. Reducing the daily suffering that I cause. It's still something even though it pales in comparison to my yearly donations but it is me living in accordance with my values and is causing less suffering than I would otherwise.

There are many more arguments both for and against that I'm not going to enumerate here, but are definitely available elsewhere in the forum. I'll also plug that I agree with Henry's comment - there are grassroots effects that are hard to measure, but my intuition is they have sneakily high EV.

But the main argument for me, and many other vegans (and vegetarians, but I'll just say vegans for brevity) I know, is that at some point, they stop wanting to eat meat. If one truly views it as a chore, it will always be seen as a sacrifice. However, if the notion of eating animal products truly grosses you out, then it's basically easier to be vegan than to eat meat.

I am not trying to deny that animal products can smell or taste great! But at this point, if I were to have a bite of chicken, the first thought for me would be "I am eating this factory farmed chicken that lived a horrible life of abject suffering" and not "hey, this tastes good." It just grosses me out. Perhaps this would be a corollary to your point 5, but at some point, the dedication to the diet becomes more internalized. At that point, points 2 and 3 somewhat fall off, because it's just a baseline lifestyle rather than part of my EA-aligned activities. 

(To be clear, I acknowledge that this isn't a universal vegan experience. I don't mean to invalidate longtime committed vegetarians/vegans who don't view meat the same way I do. I just mean to point out that there is potentially some light at the end of the tunnel that makes committing to this diet/lifestyle significantly easier. For me, that was about one year after transitioning from vegetarianism to veganism. Watching Dominion helped lock in any remaining doubts I may have had at the 1.5-2 year mark.) 

I think the question is how much you view being vegetarian as a burden/a good deed that you are doing vs just a feature of your everyday life. For me, I don't even think about it, so I don't believe that I have the "good deed offset" issue you mentioned. But others may be different!

A second question is how much weight you give to a deontological moral system being correct - e.g. you probably wouldn't eat factory farmed humans regardless of how that might affect your other actions because that seems immoral.

A third question is what would the replacement activity for being vegetarian be? Would you realistically replace that with something as comparably high impact (e.g. - eating a serving of chicken requires ~4 hours of ~torturing the chicken to get you the food - do you think whatever you would replace that with would be worth torturing a chicken for four hours? If you aren't spending the extra time donating more to effective animal charities that seems like a high bar to clear.)

Lastly, you can avoid much of the negative impact of meat eating if you eat from places here you are highly confident in the good treatment of animals (difficult to do but possible) or just eat beef and bivalves (much easier). So if you do change I'd recommend being thoughtful about it like you would with all other decisions!