First, fatigue can be caused by many things. Diet is just one factor to consider. If I were you I would rule out other causes while improving my diet.
Personally, I notice that I have more energy when I get plenty of sleep and plenty of exercise. When I have trouble sleeping (stress, sick kiddo, loud neighbors, etc.) then I am fatigued during the day. And when I am not active (swimming, running, yoga, kickboxing, hiking or doing something active 3-6 days per week) then I get fatigued.
B12 injections? I suppose they might make sense for some people but I don't get them. I use B12 tablets that taste like cherry and disolve in my mouth.
Calcium - I use fortified soymilk in smoothies and also in fortified cereal. And I drink fortified OJ sometimes too.
Personally, I don't worry about omega 3s. I have plenty of cravings for fatty things that I feel like I get enough without trying. Same goes for the other nutrients you mentioned. I don't worry about them because I eat a wide variety of vegan foods (of all colors, types, etc.).
Iron - I love beans and eat them at least 5 times a week. Iron is also in fortified breakfast cereal. I donate blood fairly regularly and they check iron levels when you do that. I've done that for years and years. I've never ever had low iron levels.
I should point out that my approach to nutrition is basically this: I do a diet journal every so often (once a year maybe) and calculate levels of this or that to ensure proper intakes. I also err on the side of overconsumption. I'm not worried about having a little belly pouch since I'm active and healthy. I don't restrict my food intake because that makes me feel deprived and stressed out - it's not good for my mental health and it's not a good example to set for my son who is naturally skinny and should never deprive himself of food. From all the things I've read and heard I think the people who are most likely to suffer from nutritional deficiency as a result of going vegetarian or vegan are people who severely limit their food intakes in order to maintain a low body weight. That's not me or my family. We love food and eat plenty of it