I encourage you to give Handbrake another look. It has one-click presets, so you could select as little as a single preset on every home movie load and then begin the process of making your final file. It's really great for what you want to do.
If you want to dig in a little deeper, you can learn to tweak a few things in a default profile and save it as your own custom profile. Then just select it every time you want to make a finalized movie. That would be:
- "open source (video) file (your edited iMovie file),
- select preset,
- "start" and then just let it do its thing.
On conclusion, you should have a high quality video at relatively small file size. It is really great for this purpose.
I use it for exactly that often (except I'm editing home movies in FCPX). But the simple 3-step process is exactly the same as I just described.
Handbrake details (a variety of individual settings you could tweak) do involve some potential learning but the online tutorials are pretty good and you can dig in as deep as you want... or not. If you want it to be "for dummies" simple, their default presets are quite good. Try a few to find the right balance of quality vs. file size and potentially just go with that.