I'm a convinced materialist, for I conceive of the universe as a vast network of instantaneous but light-like interactions reaching the present moment from all past instances that connect a present detection event to those past emission events separated by light-like paths through spacetime, so that every physical phenomenon, entity, or process that we can observe is automatically consistent with the history of the universe and the constraints in the region of spacetime that made it possible.

When idealists say that the universe is actually subjective, and that the only things that exist are ideas, I just see them reducing this vast network to those inside their brains, since ideas, concepts and thoughts would still need a physical substrate to develop, so a network of interactions (either to support physical objects or abstract thoughts and consciousness) seems, to me, unavoidable.

It's true that in my view, the reality each observer experiences is made up of unique and exclusive light-like interactions that no one else (not even itself) will ever experience again, so in this sense reality is subjective. But it woudn't be random or completely "detached" from the other undetectable realities and universes taking place in other places (in fact, all around) at the same time. Each subjective perspective of the universe contains hints of an all encompassing objective reality, since this global reality (that no one except the universe taken as a whole would experience) would be simply the result of how the subjective ligh-cones composing each observer's universe intertwine with each other.

Your subjective point of view is unique to you, but you are not the only one who makes it possible. The fact that our different perspectives criss-cross each other through spacetime is what makes them match each other the way they do, creating the feeling of a consistent objective reality beneath them.