Technically, on both platforms, you do own your content and are granting a license to the platform to display it. In fact, Squarespace’s Terms of Service claim a broader scope of rights than Medium’s do. This is also true of WordPress.com, as that is also a managed platform.
I know that it’s uncool to like Medium, so I won’t bother defending it further (I’ve been a vocal and long-term critic of theirs), but given that part of OP’s motivation is to simplify the publishing process, I think Medium is a perfectly valid option to consider here, especially since it also happens to have a built-in subscribe/email system and very easy import/export of posts. WordPress can, of course, be configured almost infinitely to do anything you want, but it’s more setup time and management hassle even if you go with their managed service.
I would also recommend a close look at Ghost since it’s a much more focused product than WordPress. Their managed service is extremely expensive though, so it probably only makes sense for those making money (or aiming to) off their publications. Unless you’re okay with setting it up on your own (the way you would with a WordPress.org install), in which case it costs whatever your appropriate hosting would cost.