A regular blood test is mainly needed just for vitamin A, vitamin D, and sometimes for vitamin B6. The rest won't silently cause critical issues if somewhat exceeded.
As for minerals, it is possible to manage the dose without a need for testing.
For B6, it's safer to supplement only the P5P form, as it's much less likely to cause any serious or lasting issue. I would limit intake of P5P to a max of 40 mg per day. If you do this, and you don't have any resulting neuropathy problems (you shouldn't), then you don't need to test it.
In contrast, the inactive cheaper form of B6 which is pyridoxine is more likely to result in problems if taken at above 30 mg/day in the long term.
I very much appreciate your advice. How would I recognize the different forms of B6 in the bottle? Would you say that a multivitamin from a well-respected company, like Centrum, would be a good choice? I'm trying to lower barriers here, for myself and my family.
Centrum is a start, but it is not great for dozens of reasons. It would contain B6, the pyridoxine form, not the P5P form, and not in a dose that's sufficient for stress reduction. I take all individual vitamins and individual minerals.
Going through your posts I see quite a few terms to google and learn about. I appreciate your advice, and for kickstarting a health improvement in my life that I've been meaning to do for years.
As for minerals, it is possible to manage the dose without a need for testing.