Take a step back. You bought a very good action. You did not list (or did I miss) the stock and barrel, but you started with a solid action. Defiance does not make junk. You want to shoot to 1,600 yards. That is a very long distance. You are talking about shooting a mile. I get that everyone has a budget, and nobody wants to waste money. But, if you are getting a custom rifle built and the action is very good to start, then work your way up to an optic that deserves that action and one where you can see your missed first shot at 1,600 yards.
Vortex makes a wide range of scopes. Their brands vary from good enough for a pellet gun in the backyard, to winning long range competitions. I am not sure any other scope manufacturer makes such a wide range of performance. What you currently have is not suited for the task. In the Vortex line-up, the minimum grade I would look at is the PST Gen II. There are some really decent PST Gen2 scopes available, although, I would encourage you to think about a step-up at some point if you are really going to shoot 1,600 yards.
What you will soon notice - if you have not already - is that the members here both are very knowledgeable about shooting and about scopes, and that they are very opinionated. We all come to this forum with our biases. Scopes are probably the most personal of choices, and members here love one brand and hate another, and much more so than opinions on actions, chassis and barrels. So, you have time to learn, and if your budget dictates that you get by with what you have until you can afford it, so what? Get the experience behind the trigger, and see what you like and don't like about the scope you are stuck with.
What you will notice is that shit scopes look okay in 1-4x or even up to 500 yards. A crappy scope at 5-25x just makes no sense. For me, at admitted scope snob, seeing a crap scope sold for 5-25x long range shooting is like fingernails on a chalkboard. (Gen Z folks, make your interpretation since you probably have not seen a chalkboard) But, there are value brands out there that have surprised me. Athlon is one, for sure. I am not an Arken fan, but perhaps I am bringing my bias in. I like the Burris XTR3 in mid-magnification (3-18x), and I know there is a longer range model. I do not have first hand experience with it, but perhaps that might be on your list. I would not rule out a Nightforce NXS 5.5-22x, and look hard at the Vortex Viper PST Gen 2 5-25x. I am more of a fan of the PST scopes in lower magnification, but with cost in mind, this would be a good candidate to keep on the list. I hesitate, but I would at least consider the Nightforce NX8 - either 2.5-20x or 4-32x. Price jumps a bit, but keep in mind the rifle you are building.
Scopes are like shoes. They are personal and you need to own the decision. Also, consider the reticle. You will be spending time behind the reticle. There are some reticles that I cannot enjoy, but others swear by them. Also, when looking at scopes for 1 mile shooting, do not think about how the target looks. You need to know how the dirt and grass looks at the target. What sets the great scopes apart from the okay scopes is the ability to see your miss hit the ground, so you can adjust for the next shot.
Good luck with your project.
