The definitions of several concepts are fuzzy, and therefore can be circumvented or challenged or abused by all sides of the equation.

They are, but it’s not like they’re very definite nowadays either.

What is a ‘similar product’ that is allowed after 30 years (and therefore what is a ‘dissimilar product’ that would be forbidden before),

I’d say “similar product” is anything that doesn’t try to pass off as the original one, and is mechanically different enough. Palworld for example, or all the other Pokéclones that popped up in recent years.

how would a non-profit that just pays high salaries to its managers fare between the marks of 30 and 50 years (and just gives some little money to research or charity).

They wouldn’t, in that period I’d allow stuff like piracy or free cultural events, stuff like that. Obviously the copyright holder would still be able to profit off of their own products, but everyone else would have to ask them to do so.

And again, why give artists and creative companies so much more time of IP protection than we give STEM inventors and companies time in patents (this random site claims patents last 15 to 20 years only) ?

Because those are things that humanity needs to progress. I do think they could be longer in a different way, like “they can be used by anyone without consent from the inventor, but they need to pay a small percentage in royalties” or something like that, just to ensure they have a permanent source of income that’s enough to live off. I’m not knowledgeable enough about that to talk though, so I can’t really answer that question without going into baseless speculations.