So let’s say I were to eat a diet very rich in viatmin C and low in potassium for a week, then a diet very rich in potassium and low in vitamin C for another. Would this still be considered well-balanced?
No. It’s not going to kill you or anything but it’s not optimal for your health.
Vitamin A, D, E & K you can be chronically deficient in but because they are fat soluble averaging the RDA over a couple of months is generally sufficient. Vitamin C you need to replenish every three weeks.
B2 it’s good to eat every meal because your body doesn’t store it at all. B12 you could stop eating right now and it would take a couple of years for symptoms to show up at all.
Minerals your body can leech most out of your bones but that’s a really bad idea as it will weaken your bones and there is a lot of waste in the process. There are a number of functions, like muscle development, that have a greatly reduced function if your dietary needs are not met too.
Aminos you need every day, they are not stored. LA & ALA are essential but your actual dietary needs depend on your previous diet and if you have a calorie deficit or not because they are stored in adipose.
All of this is about continuing to live rather than thrive. B12 might take years to deplete but if your diet is high in B12 your immune system will function better.
There is also the isomer effect. Different nutrient isomers are more effective than others, the more reactive ones tend to be better but they also don’t store. The L isomer of vitamin C we don’t store at all and it’s the one that gets destroyed by heating, eating it lets your body skip a couple of synthesis steps.
Variety ensures that you are getting a good supply of nutrients, with various isomers of those nutrients so your body performs, optimally. Your vegetables should be a rainbow and this is not very difficult to do, if you are having mash instead of just potatoes add a bunch of other starchy root vegetables. If something calls for onion use leeks and onions.
Some nutrients are stored, some are not. It’s hard to avoid minerals tbh, so potassium will probably be there, but vitamins can be scarce depending on the foods you eat.
Vitamin C and fat soluble vitamins are stored so you don’t have to get your daily recommended dose every single day,just on average.