I think multivitamin products are a bad idea generally. All those nutrients together, in fixed doses. And many of the ones sold at drug stores (here in the US) are trash quality with lots of toxic additives. I avoid them like the plague.

Fish oil and EFAs are tricky. A low quality product is worse than nothing at all. Last thing you need is an oxidized or rancid fat, or a product with lot of impurities. It's also important to get ratios right -- EPA, DHA, GLA, etc. And there are different sources, some animal some vegan. Dose matters too. Nordic Naturals seems like a good brand.

I think Magnesium is a no brainer. So too D3, K2, maybe C, maybe A, maybe Zinc.

Probably most people have terrible gut health. It's worth experimenting with digestive bitters, enzymes, HCL. Probiotics not sure, but eating fermented foods could be good.

B vitamins, have to know what you are doing. Dose and the right form are critical it seems.