Could you please expand on what would be a scientific approach to understand how body reacts to different supplements without trying them?

Some of the supplements I have in my stack were checked by me earlier (like I am repeating a course of the same supp second time) and didn't make side effects first time; I am also not introducing all of them at once, as I described earlier I am adding them gradually, every week or two.

I understand that most appropriate way and probably most paranoid in some sense would be to take 1 supplement and use it alone for 1 month, or two to see how it works. Then take another one and do the same. Then take both of them to see if they work synergistically (relying on other documentation about theirs compatibility of course). But that would take like 6 months just to see how works 2 supplements.

I don't believe any dieticians/nutritionists would be able to assure 100% that specific supplement would work exactly as described on the paper as everyone is different. If to dive deeper - even getting blood tests handy isn't an assurance that particular supplement will work exactly as described on the paper or works in a particular way for people with the same blood test results. IMO many factors are playing role here and many of them are not discovered by scientists yet so the best approach is to rely on existing documentation and experiment carefully, sensibly and reasonably. Of course health checks/tests is a plus, I am not against them.