Have you considered not taking anything? Most studies show multivitamins have somewhere between no effect and a slight negative effect except in groups with special nutritional issues. It might be possible to optimize further with the right combination (if the lack of effect from multivitamins is because they include both beneficial and harmful chemicals with a net zero effect) but I don't really think we know enough to do that right now.

Every multivitamin I've had is at least 2mg Manganese/pill. Cut the pill in half and it's half as bad for you?

I also suspect Vitamin A is similarly harmful in isolated form compared to the much more complex natural sources of it but don't really have any evidence on that.

I would bet against the specific prediction without rejecting the principle. That is, I expect when maximising a given function along the lines of "useful when you don't have enough and minimally harmful when you have more than enough" there is a specific isolated form that is better for supplementation than complex natural sources. Where my in principle agreement comes in is that I consider it highly unlikely that the actual isolated form used in an arbitrary multivitamin supplement to be the most desirable one.

The most obvious reason I would expect to find that at least one of the forms of Vitamin A to be better supplemented in isolation than just adding more of a combination is that some of the forms are largely inactive until they are converted by the suitable enzymes. That gives you a potential rate-limiting buffer in the case where you are already eating too much of the stuff and adding more would otherwise be bad.

IIRC at least some multivitamins contain at least part of their Vitamin A as carotene rather than retinol.