First of all, keep your notebook. That' not over-thinking. It's just good practice. And look back over it once in a while, so you can learn something from it.
I have two thoughts on changing exercises. Or more.
First of all, it is possible to either change exercises too much, or not enough. Too much is the guy who is jumping to a new workout everytime he sees a different article in T-Nation. Not enough is the guy who stubbornly refuses to change even when he's not making much progress, or worse, when injuries are accumulating.
The other is that there are two kinds of changes. One is just a one-time thing for something different, or to try out something you haven't done before. The other is to make a strategic change in your lifting template. And those changes can be small or big. Make small changes fairly often, big changes less so. Not that I type all of this it sounds obvious, but I don't mean to sound patronizing.
For instance, you can throw in a few sets of a different variant (DB instead of BB bench, or vice verse) just to do something different today, but then sticking with what you had been doing as you continue. But at some point you may decide that the DB would be good for you in the longer run, so you put that into your routine for a few weeks. That's a small change. Or to decide to do chins before bench instead of after. That's a small change.
A big change is a major rearranging of your template. You should do that once in a while, on the order of in a year or two.
Other than the notebook, that was exactly what I was talking about. I'm still going to follow the ExRx 2 day, push / pull split. Sometimes I change the order, but only to save waiting for something. I've tried other templates from other sites years ago, but none compare to this one for me (and it also makes the most sense to me).
You certainly don't sound patronising. You echo what I was thinking but put it in another, concise way. I especially like "Make small changes fairly often, big changes less so." I couldn't have put it better and that's what I was hoping to hear. Thanks.