I think Sanny uses a Turner King Khan to pretty devastating effect on his mountain runs. If you can find something similar for your budget (full suspension, 4″ tyres) then go for that, otherwise for big mountain riding I’d value suspension much more than fat tyres and go for a 6″ (ish) full suspension bike.
I currently use a Commencal Meta AM V4 which was about £1500. Weight is about 31lbs, which is less comfortable than my old 26lb Specialized Stumpjumper Evo but still bearable. It’s about the same weight as most fat bikes. I use adjustable travel forks to get the front down.
1x is fine if you’re reasonably fit- you’ll be hiking up a lot of the best ones anyway. I’d go 1×11 if you can, or get an expander kit for 1×10 to take it up to 42t. I run a 30t chainring with an 11-42 cassette.
That’s assuming “up and over” the mountains stuff, aiming for the summits, if you’re sticking to the lower slopes and going through the glens then a fat hardtail would be fine.