getting hold of salsa/surly stuff seems hard work and expensive!
Deffo go the Sir 9 route then 😉 You can also add in a rigid carbon fork with mounting-points. Keep the rubber thin so the magic shines. I have a bikepacking rig in modestly-butted 4130 cromo flavour, and a touring bike in hand-built Reynolds 531ST flavour. The ride of the bikepacker is cushioned by up to 3 inches of tyre rubber. The custom 531 touring tubeset OTOH is running on 28c tyres and I feel all of that steel-magic all of the time.
I enjoy both rides in different ways, and they both carry a shit-ton of luggage when required. The big rubber goes anywhere in great comfort, and the long slack frame is a lovely land-boat that floats over root and rut. The big rubber is insulation where suspension may have been. At the other end of the scale the Reynolds 531ST on 28c has a beautiful zingy ‘buzz’ feel that is quite sublime. I can only imagine how it might feel with 2.4” of tyre. Swimming with socks on?
It’s also worth weighing it all up and thinking how much of your hard-earned you can/wish to spend on shaving a kilo here and there before adding a litre two of water and sleeping-bag to the mix…