The whole nature of this thread kind of changed when you included the backround information. You have been using a top of the line FX, you really liked it, you called Utah airguns spcifically looking for a high end airgun. So, you are not worried about the money, you know that you really like airguns. You already have an Arken scope, so you know what decent turrets are and what FFP/SFP is.You are already in the rabbit hole. You eluded to the fact that you liked the DRS Pro better than the Impact because it doesn't have so many adjustments. Lastly you mentioned that you didn't want to buy something that you would feel the need to upgrade in a few years. I don't know you, but I know that everyone that I do know, that considers to buy top tier stuff, in any hobby they are involved in, is never content when the new better stuff arrives. The good thing about FX is that there will be an upgrade path, road, super highway. All the other brands, less so. There is a lot to consider with the brands and platforms that hinge more on your personality than anything else, and what you find important. If you just want a premium airgun that is simple and robust to the extreme, you should look at the American Air Arms Evol/Paradigm too.
Caliber, these days, with bespoke guns doesn't matter much unless limited by law. Same concerns, safety, economy, effeciency, precision, availability, legality, lethality. What people are doing with .177 slugs lately is pretty amazing.
The really hard advice to give, is whether to use slugs or pellets. To be clear, I don't own an FX, but I would, if I was going to be building a slug gun, today, with what is available right now. Simply because there are a lot of options to get it right, if it doesn't show up that way. You want a gun for ground squirrels, so shooting up isn't such a concern, and you will be shooting mostly further than 80y, so yeah slugs. A dedicated slug gun will most likely be a one trick pony, but if it is a good trick and you are fine with that you're aces. Slugs can be tough though, and problematic, with supply or design changes or barrel maintenance, but mostly just finding the right one. Pellets are pretty easy by comparison, and for all intents and purposes just as good or better to 100y if you get the wind right. Shooting pellets will give you a lot more choices on which gun to get for sure.
Finding and shooting a great slug is awesome because it is still rare, at least it seems to be from all the posts on this forum. For pure pleasure and best all around shootability my totally biased favorite projectile is the .25 33.95gr JSB. Opinions vary.
Happy hunting!
I’m liking all the posts I read as I get to them. Just busy with harvest and my other job at the moment so it’s not as much activity on here as I would like. 
