Multivitamins may actually be harmful. Fat soluble Vitamins may have weird properties which could be bad even with correct dosage. Vitamin B6 can be easily overdosed as even the recommended values are questionable.
However, I strongly advise against going to just by “terrible diet” and do only until something shows on regular check-ups. There can be subacute deficiencies, they won’t show up on routine checks or not at all anywhere as we can’t “Star Trek Health Scan” the human body and especially the brain. Also your diet is terrible by default. Why? Because depending on the geographic region and especially with modern agriculture which killed off all kinds of biodiversity and trace elements is more than likely to be worse than a century ago with the same ingredients. It is not easy to recommend a Vitamin stack as there are so many things we don’t quite know and regulation is practically absent.