Spice is not a solution in dune in fact the whole 4th book and the end of the third are centered around forcing humanity to wean itself off spice so that it may evolve.
The central concept is that humanity must not depend on machine or drugs or complicated eugenics and must instead look inwards and improve itself by facing hardship.
In foundation (at least the start) the complicated maths is essentially there to prove that all establishments fail and survival requires constant change. Very differently from dune foundation sees technological superiority as key to this and importantly the ability for society to change in order to support the technological progress.
Even if you don't agree with the above neither book aims to "fight imperialist bullshit" if anything they both quite staunchly support the idea of a benevolent dictator controlling all.
Or is Dune about the folly of different types of dictatorship; sadistic, benevolent, religious or machiavellian? Taking only the first book (because that's as far as I've read) every leader is thwarted or confined by the consequences or weakness of their own style of leadership.
I read an interview where frank said that his intention was for Dune to be a cautionary tale about the dangers of charismatic leaders (which is to say, the "classic" hero archetype). Which - for the first book - tracks pretty well. The free are basically just used as cannon fodder for Paul to win back his power (and a lot more), then when he wins, he sets them loose on the universe because he can't control them.
The trouble I have with that though is that he goes on to contradict that point in later books, but I won't get into that because I don't want to spoil anything for you
It's honestly crazy how many people can read Dune and completely misunderstand the themes of the book.
Though to be fair, it sometimes feels like Frank himself didn't fully understand what themes he was going for. Books 1-3 were staunchly "Beware of heroes, charismatic leaders will lead you to evil and despair", then in GEoD, we find that literally the only hope for humanity was millenia of oppression by a totalitarian government.
But either of those two takes is still wildly better than "spice saves the universe" lol
Everyone was out for themselves or their narrow view of what was just and best for humanity from their simplistic and self-centered perspective.
Leto 2 was the exception because he was out for his narrow view of what was best for humanity from his broad, self-centered perspective that still didn't really lead anywhere.
The actual point of the books is that no ideal survives the test of real time, and over time civilization tends to ossify, so we are doomed to catastrophe by our very nature.
In the non-canon book Psychohistorical Crisis, the Dune universe is part of the past of the Foundation universe. The Fremon are known as the "Frightful People" to historians.
