I will offer a few observations on some of these bikes. I too have an El Mariachi and love it for everything. The versatility of the design is fabulous and it is very comfortable for long days in the saddle. It is my bikepacking setup presently but I take it out to the local trails pretty regularly. Current setup is a 3.0 Knard in the front and a 2.4 Ardent in the back with a rigid steel fork (Firestarter) and a Jones Loop H-bar. But I have had it set up a number of different ways and not even scratched the surface of what is possible. So. Much. Fun. So, I would agree about adding it to your list.
The Surly ECR is a dedicated off-road touring rig and from the reviews I have read, people don’t necessarily like it for regular riding as much. The bottom bracket is quite low and it doesn’t lend itself as much to unloaded trail riding. Beautiful bike though. I just think it is more specialized than you may be looking for. Here is a link to a comparison of the ECR and the Krampus:
http://www.pedalingnowhere.com/gear/surly-krampus-vs-ecr/The Surly Krampus on the other hand rides the same rubber as the ECR but is more oriented toward trail riding as far as geometry goes (higher BB and all that). And plenty of people use it bikepacking as well with great results. I think in contrast to the ECR you could run different bars and/or a few mods to make for a more comfortable long distance rider and be more successful than trying to make the ECR a daily trail rider. I would keep it on your short list.
The Horsethief is a bike I have admired from afar along with the Spearfish. FS is very comfy in general if you have not ridden them before and this bike has a reputation for going the distance. I think it would make for a great touring rig. The downside being that with full suspension you do not have as much space for a frame bag or numerous bottle mounts. You also have more moving parts which for some is a downside if heading out to the hinterlands or a setting where a blown shock could create big problems. That may depend on how long you envision your bikepacking adventures to be and how far from civilization you are headed. But this bike is well spec-d and so sports reliable componentry. In addition to the El Mariachi, I also have a full-suspension (Giant Anthem) which is a recent acquisition and my first FS bike. Going from a totally rigid El Mariachi (and having a bit of a purist snobbery) I was expecting to be more underwhelmed than I am. I love my full-suspension! Confidence-inspiring, very comfortable, surprisingly fast, nimble, etc. But I also love my all rigid El Mar. This is why you need more than one bike. Just sayin’…I have 4.
I have enjoyed riding some 29er+ rigs in a variety of settings. Super super fun setup and I am now a convert to the ways of 29+. But I am not sure I am ready to commit to any true fat bike as a FT rider. It depends on what you like to ride, but they seem to me like more of a specialty item – overlanding, sandy settings, beach riding, snow riding, etc. Regular trail riding, I’m not sure (unless you live in one of those more extreme settings). I like speed and my limited experience with fat bikes were not fast rides. They WILL ride over anything and are a great choice for select environments or trekking along the Alaska coast for example. Very stable and grin-inducing rides. But if I were looking for a regular rider, I would not choose a fat bike. Opinions may vary.
And Vikb's point is well taken. Despite the time I devote to planning and dreaming of bikepacking adventures, the vast majority of my riding is regular local trails. It does make sense to decide based on that to maximize your enjoyment. Better to have to adapt your regular rider for a bikepacking adventure a few times a year than have to adapt an off-road touring rig for regular trail riding every week.
