I like the 6.5 but my first rule of firearms is I don’t buy a caliber that ammo isn’t cheap and plentiful at Wal-Mart. (this means it is widely used and available everywhere) - that’s why I stick with the 308 which I’ve learned to shoot quite adequate for any needs I may have. I really like the idea of a pistol caliber carbine and pistol of the same caliber - so you can focus on stocking up on a single caliber of ammo. A 357 lever action and revolver have been a traditional choice but if one is starting now a 9mm handgun and one of the increasingly popular 9 mm carbines is a good choice due to the wide and cheap supply of ammo. Of course a shotgun is essential. Having interchangeable 18” and 22” barrels is a plus.
That raises an excellent point. During the last ammo shortage under Obama, you couldn't find 9mm at Walmart (where I lived, at least), but you could often find .40 S&W. During that same period, Walmart tended to have zero .223 or .308 on the shelves, but did have 6.8 SPC, .300 Blackout, and steel-cased 7.62x39. So, one possible approach is to have a handgun that will fire 9mm OR .40 S&W, and a rifle that can handle a NATO cartridge OR an alternative. It's often as simple as swapping the top half of the firearm (and maybe mags), and the additional parts usually cost less than buying a second gun...