-
This would be less gnarly stuff bordering on gravel I suppose where you don't need the suspension. Main thing is the amount of luggage you can take on a rigid fork, but also the efficiency, and you need to take more care of a suspension fork
frankenbike-
In my experience efficiency gains would be minimal, as most forks can either be fully locked out or stiffened through damping, and service intervals are going to be effectively increased due to the very light usage, beyond just lubing the stanchions every so often. There are also bolt on luggage options available, though you can fit an awful lot between saddle/frame/bar/feed bags. I also don't think the slackniss is going to make much difference once you're cruising along, aside from a bit of bar flop while you're near stationary. Sagged head angle for 65deg is going to be about 66.5, which is the same as my Swarf. Big downside is really only the weight.
Realistically the longest fork you're going to be able to swap for a rigid is 120mm, Unless you splurge on a Whisky No.9
TvH
Why can't you just tour with a sus fork? Maxi comfy. I've done everything from bike parks to multi-day bikepacking on a modern full sus. It's tyres wot make the most difference