28 Nosler ammo may not be cheap but many calibers aren't nowadays. As for availability Ammoseek has over 500 vendors selling 28 Nosler ammo. Prices start around $40 and go to Over $80.I wouldn't touch a 28 nosler personally. Ammo is tough to find and expensive when you do find it. Of those I'd go 280 ai. Honestly though, you basically have the bigger stuff covered. Have you looked at some of the smaller cartridges? I'd look at 6 creed, 243, 22 creed.
Christensen Ridgeline FFT 28 Nosler or Weatherby 307 Alpine CT 280 AI.280AI. I never discourage someone from wanting another firearm. Way worse things you can waste money on and the 280AI is a damn fun cartridge to shoot. What rifles are you deciding between?