Yeah you don't HAVE to change stances at all. You can use the default one (stone) to kill any enemies, at least that I've encountered. You encounter plenty of guys with shields before you even get the water stance, that is the one recommended for shield guys.
I do find it a bit confusing trying to do all of the stuff mid-battle though. Even when not battling and I'm switching stuff I mix up L2 and R2 and the Dpad and face buttons constantly.
Holding L2 brings up 2 crosses, where the dpad changes between 4 options, and the face buttons change between 4 options. Holding R2 does the same thing but with different options. It's just a lot of stuff to manage. It's done pretty well and is fluent, but just a bit overwhelming to try and get used to.
I haven't played it since Saturday though. It's cool and I like it but I'm not having that urge to play it like I did TLOU2. That has always been my case with most open world games though and why I prefer linear story driven games. I will probably get back to it on Friday.