@NotSavvy
Dave, you asked for advice. Here's mine. Forget File Vault (FV).
Firstly, your iMac is Desk bound in your home - you don't travel with it.
Secondly, unless you have secrets the CIA or Kremlin want, the Mac's own built-in security is perfectly adequate.
Thirdly, FV is only "operational" ie active when you shut down the Mac - obviously it cannot function while you are using it. Shutting it down each time you leave the Mac is not likely to appeal to you.
Fourthly, if you ever forget the PW and lose the recovery/security key, nobody, not even Apple can get into your iMac. It's dead meat.
So be sensible in your practice and stop worrying
And - if your wife's useage is limited to the occasional print request, no need to add her on as a second user.
But - there is some sense in creating a second Admin User Account and having done so, leave it alone. Why? Well, if some faults or problems develop on your iMac, having a "Clean" Admin user account can come in handy in determining the nature of the problem.
Ian