Hi, and welcome to the forum
You certainly bought the right bike for the type of use you give it - The LTS isn't exactly known for being fragile and was designed with the type of riding you do in mind!
I'd suggest that you look at replacing the rear shock at some point (if it's still working fine, then there's no need to do it yet, but replacement parts are hard to come by). The only problem with the later LTS' (like yours) is that the rear shock has a trunion mount, so there are only a few shocks which will fit it ( Risse Racing make some nice ones
) - you could also look at buying a Fox (or similar) coil shock and having a new trunion made to fit. Unfortunately, the Rockshox shocks were never the best and there are less and less of them coming up for sale. I've got 2 spare ones though if you're ever stuck
I'd actually stick with it as it is - or maybe change the forks for something with 130mm travel ( Bombers are always a good bet and there are often one or two nice pairs for sale on this forum
If your back is getting sore, then it sounds as if your either too stretched out or the front end is too low. A stem/handlebar with more rise, or a shorter stem might ease the issue.
I'm actually building up a STS at the moment, which I intend to use for almost EXACTLY the same as you! viewtopic.php?f=21&t=250479