Commerce is definitely Craft's achilles' heel right now.
Like you, I'd love to switch over completely off WP but picking the best tool for the job means either rolling up your sleeves and writing your own or picking the best available solution and unfortunately that could be WP/Gravity... at least right now.
We're also hoping to hand off the form creation process to the client afterwards.
Not to go too far off the Craft path here, but from a client services perspective, this part scares the hell of me.
Forms tend to be tricky beasts to begin with—especially so when you add in the commerce part of it.
I don't know what your registration forms look like but I'm guessing a lot of them are going to be very similar. Do you actually need a form builder to put in name, address, etc. Or is the only thing that changes on the form the retreat date, lodging location, etc.
You can do a lot with Craft's custom fields and instead of building up a form like in Gravity or Ninja (like you might do in WP), bake in those fields that won't change in your template and just let the client change a few pieces from the back end—like the date, price, etc.—and compose the rest of the form with Twig and an action handler.
Regardless of whatever technology you pick, this kind of project tends to need ongoing support as well so if you can do it, get the client on some kind of monthly retainer; that'll remove a lot of the uncertainty as you move forward. Sometimes what clients say they need and what they actually end up needing is completely different. Craft is going to grow as a platform too, so basically you're buying some time as the client grows with it. It's a nice sentiment to think you just "hand it off" but in my experience, it rarely works like that.