Conflicted about whether to start preparing for grad school
A 3.1 undergraduate GPA is indeed low (even from a top school) among successful applicants to a PhD program.  But there are over 130 R1 universities in the US, so it seems likely someone will take you.

I can't advise whether you should get an MS and work for a while before thinking more seriously about a PhD.  What I can tell you is that very few people who start with that intention actually do it.  It's possible to get a master's part-time while you're working but realistically, a PhD requires a full-time commitment and therein lies the problem:  Once they begin working full-time, pulling in the typical $100K+ engineering salary, most people begin spending it.  Worse, they get committed to continuing to spend it.  They take on loans for cars and condos or houses and so on, and then they're handcuffed.  They can't easily leave.

In my own 40+ years in industry, I've known lots of engineers with PhDs.  But I can only recall one person who quit to go get one.  And he had only been working for a year and hadn't yet started spending.  Sure, there may have been others, but I can't remember any others.

So, if you are thinking about a PhD, the best time to do it is probably immediately after you finish your bachelor's.  When I advise engineering students here at Michigan, I point out that I wish I had done that.  I never got a PhD and I regret it.  (And at 68, I think that ship has sailed.)  YMMV.