Seems to me you're spoilt for choice! So Many Good Bikes!
Any one of them could do your trip!
I don't own a 990 but many riding buddies do and I've ridden a few.
The SMT model would not be my first choice mainly due to the 17" front tire and short fuel range. 990/1150 Adventure may be a better choice?
But if you feel the SMT cast wheels are tough enough to handle hard rock hits/massive pot holes and if you're good enough to ride a 530 lbs.( 240 kgs.) bike through deep sand / mud on street tires ... then you're all set!
Seems that a loaded up, tall-ish bike would be a bit of a handful? Probably add 100 lbs. with gear, Boxes, racks, hardware, tools, guards loaded on board.
KTM's are great bikes, super fun to ride. (I've only ridden the 990 Adventure and 950SE) Both too tall for me, I could never travel loaded up ... too many tip overs!
I guess the huge cost of shipping your bike from Oz to S. America (and back) is not a problem? Have you got that figure?
Of all your bikes I would be most comfortable on the Tiger 800 (XC model?) or the Pegaso. Also consider how much you could lose if something goes tits up. The Pegaso is not super expensive ... but the KTM and Tiger are. Not sure any Oz insurance would cover you and local insurance is dodgy at best.
If you were to travel mainly in Chile and Argentina then the SMT would be great. Fantastic paved roads on the main routes. But as you go North into Bolivia, Peru', Ecuador and Colombia you may find rougher roads if you go even slightly off the beaten path.
Even "Main Roads" can be rough, rocky dirt and in bad weather may be a challenge. Sure, if you stay strictly on the well paved Pan American, no problems, but doing a bit of exploring is part of the fun.
Keep in mind in some places road speeds are low, so you can't exploit the KTM's massive power. In more Urban areas speeds are more like
40 to 50 kph. and in some areas lots of cops waiting for foreign riders to come by. Lots of riders buy inexpensive 125's or 250's locally and tour on those. They can be sold off (legally) at trips end. You could sell your KTM too ... but could be a bit more of an effort and not strictly legal.