Of course veganism is self-improvement. It's healthier, more mindful, better for animals, better for the environment. I cannot think of anything I have done in my life that has improved my wellbeing more than becoming vegan. Sorry, if you don't like that. It's not a criticism of your lifestyle choices, don't take it personally.
I’m going to go through this one by one.
Healthier: not necessarily. Yes, if you eat no processed foods, in a balanced way, in enough quantities, to ensure you consume adequate calories and nutrients, it can be healthier. But it’s much more time consuming and expensive to achieve this as a vegan than as a vegetarian or an omnivore.
Mindful: what do you mean? That you think about what you eat more than a vegetarian/ pescatarian/ omnivore? If so, do you hear how arrogant you sound? Not all non-vegans mindlessly put terrible food into their bodies all the time. Arguably, it’s much more mindful to ensure you’re a responsible and healthy omnivore than a vegan. Vegans can mindlessly reach for anything that says “vegan” on the box. Pescatarians may well be researching seasonality, sustainability, origin etc.
Better for animals: I agree with this more than I disagree, so yes.
Better for the environment: are you sure? Do you think it’s best for the environment for all 7bn humans on earth to receive their calories and nutrients from plants? Think about that seriously, in terms of water usage, seasonality, transportation, urbanization, gloabalism. You’ll see the answer is most certainly not. So only some people can sustainably be vegan, for “the environment”. Are you claiming that saintly title for yourself, or do you think other people (say, in developing countries) claiming their fair share of resources from elsewhere is what allows you to be an “improved” human as a Western vegan?
Your well-being: wonderful. Very happy for you to have this private joy. Doesn’t allow you to claim the title of “self-improved” as a consequence.
Criticism of my lifestyle choices: the thought hadn’t occurred to me. A vegan’s choice to be vegan has no bearing on my choices. Just like you choosing to wear brown shoes today has no bearing on my choice to wear black boots. You’re not an improved human being as a consequence of that choice, and neither am I a lesser/unimproved human, that’s all.