I totally agree. I do the same thing by manually putting in the old email and typing in at least a basic pseudo-header like “On X Date, Y <email> wrote:” to artificially create a thread. Like you I value the ability to track opens and set follow-ups enough that I do it, but it’s crazy that they don’t have such a basic standard email function that’s been around decades in every free email program and it does indeed add HOURS to my workflow each week.
Unfortunately, I can’t get the benefits of tracking etc. using my desktop email client because I’m on a Mac and they only have a PC Outlook extension, so I’m stuck (even if I’m willing to use Outlook on Mac) using their app-based or web-based UI for this. Please fix it!