You do not understand how resources in space work.
For anything other than organic matter, there is more of it in space, than at the bottom of planetary gravity wells. Asteroids, planetoids, comets, there are larger deposits of any industrial metal, heavy metal, or precious metal, than you can wrap your mind around out there.
And whatever tech level you are at (unless you follow the very specific sci-fi principle of using things like Stargates to travel directly from one planet's surface to another), it is easier and cheaper to find and harvest such materials in your own solar system, just floating around, and then harvest and refine them, than it is to go all the way to another system, go down a gravity well, fight someone else for them, drag it back up out of the gravity well, and go home.
To try to communicate to you how sharp the disparity is, the moon is on average about 380,000 kilometers from Earth. Once you're already in orbit, it is comparably easy to travel to and from the moon, as it is to cross 2000 kilometers of ocean on the surface of the Earth. This is a very rough equivalency, but once you are up and out of the gravity well, things are so immensely much easier.
You like Gold? Platinum? Uranium?
How would you like an asteroid or planetoid with solid chunks of the stuff the size of mountains? It's just sitting there, waiting for the taking, nobody's going to fight you for it.
Oh, you do want biomatter? Well, you're in luck, because sunlight is free in space (sometimes painfully so), and water ice is also pretty common, especially in comets. Pick it up as you please, and hydroponics for whatever kind of life you already have is fairly easy. You can also electrolyze it to separate the Oxygen out if that's what you need to breathe.
The Silicon and Carbon that make up the key elements of dirt can also be found in space, they're not exactly uncommon, so you're good to go there too.
Bottom line, if you are technologically advanced enough to travel interstellar distances, you aren't going to have any need to conquer other people for raw materials. There may be all kinds of other reasons, but the Romans would have been better served trying to import Iron from Hawaii, than aliens would be trying to use Earth as a resource mine.
And you don't understand, simple laziness and cultural habits. Yes, it's all cool, only that you have to work at it, you have to organize it.
And here, you're conquer weaker race, forcing them to work for you and only reaping the profits.
If they rebel, you pacify them. There's a reason I said the aliens would take the easiest path. Because it's easier to force someone else to work than to earn it yourself.
The same thing happened on Earth for centuries, why bother extracting raw materials, when all you have to do is conquer those who extract them?
Anyway, this whole thing assumes one important mistake. Why do you think technical sophistication precludes conquering others for resources? When precisely the simplest way to get resources has always been to conquer and steal from the conquered.
Especially since you can use a conquered race in all sorts of ways. For example, force it to exploit its entire star system. This way you increase the profit of a particular system.
We are too concerned about the cost of interstellar travel, because for us it is a huge scale.
For a space civilization? This one, by the time it leaves its star system, will probably be so proficient in the subject of space travel that it actually won't even think about it. Ot, operational costs.
Note that as a rule, few people try to find stray resources. Much more common was the seizure of existing deposits.
The conquest of the Americas is a good example, first occupying areas already inhabited by civilizations already having extracted deposits. Only then did the expansion go into sparsely populated and empty areas.
And this is more how space conquests will look like, first one will look for a reasonably advanced civilization in order to, based on its planet, organize mining in the system.
Using already existing resource extraction to facilitate the search for and extraction of resources that for the time being are lying idle.
What you're proposing works when the space system is empty or civilization is so primitive that it's not worth bothering with. Actually, it's even doubtful that it will be noticed, after all, you don't pay attention to ant colonies, do you?
But once there is something serious, it is better to eliminate the competition. Because, after all, you'll be in the system for centuries, during which time an alien race might evolve enough to try to snatch up your resources. Without realizing it's doing it.