As someone who liked the book more than the movie, your first impulse was correct but the movie should just be considered its own thing.

You are half correct. The movie is essentially a parody of the book. The book is unironic.

In the book, we have no reason to believe the war against the Psuedo-Arachnids is anything but defensive.

Heinlein was relatively fresh from the WW2 US Navy and definitely has this weird warrior-poet fixation on the ideal modern officer, and blathers on about it quite a bit, but that alone just makes him a militarist.

Where he goes off the rails is his concept of "service guarantees citizenship," but unlike the movie it is very clear in the novel that "service" includes not just military personnel but civil service, teachers, fire fighters, etc, those who can broadly be said to contributing to the public good.

That's pretty suspect, but not nearly as fascist as the movie makes it seem, especially with their hints that the destruction of Buenos Aires was a false flag. Verhoeven rather famously didn't read the very short novel he based the movie on, which is why we don't get Power Armored Shoulder Nuke Space Marines, the cheap illiterate coward.

So, in short, Heinlein was a militarist liberal, self described as a "radical liberal," thus his generally positive take on libertarianism later in life. In other words, basically just a neoliberal ahead of his time.

Some might note that means you only have to cut him to find something else though.