You're right - it's a battle of wills, and you're losing. Don't insist, don't bribe, don't punish, don't cajole, don't threaten. Don't engage at all. This may well be a bid for attention, and she's getting plenty of it. Think of the parents with 7 kids - they are not short-order cooks. The put the food out, and that's what there is. Anyone who wants to have a tantrum is free to have it in the safety and comfort of his or her own room.
If she's not hungry, she doesn't have to eat. Excuse her from the table and put the food away. She will not starve. Disengage from the demands and the crying fits.
You might try a divided dish with sections - I've seen some people use an ice tray with compartments. Put an assortment of foods in each one - some yogurt, some hardboiled egg, some sliced grapes and apples, some pasta or rice, some cooked shredded chicken, French toast cubes, whatever you have. If you are making something for the rest of the family, use those ingredients separately for her. You can make it fun with her own utensils or some fancy toothpicks if she can use those without poking herself.
Some kids have texture issues vs. taste issues. But a divided plate will help her make choices. If you are giving her the same foods you have available for snacks, you aren't really catering to her in terms of preparation or selection, just presentation.
Don't push the gummy vitamins - they have very little nutrition in them and are poorly dissolved and absorbed. If you feel she needs a supplement, then you should look for a powder form that you mix in liquid and that has at least 60 ingredients (nutrients, not chemicals) including protein, many more vitamins and minerals than those gummies, DHA for brain development, and that is formulated by a pediatric food scientist. I can recommend one if you want to go that route. It's a complete food and will make up the gaps in her diet (which every kid has, every adult too), and then the rest of what she eats is "extra". That will give you some reassurance while you work on the attitude and a calmer environment.
Barring any major medical issue, she's not going to starve. She will eat when she is hungry and when she stops getting a payoff for this behavior. You cannot make her eat. But do not reward her by letting her eat, say, fast food just because "it's something".
I had a very picky eater. I disguised a lot of food and have great recipes for things you can do if you want to. But the main thing is to take all the fussing out of the environment.