Squarespace is awesome, but until they offer SSL certificates I can't recommend it to anyone. This is very unfortunate because it's a great product otherwise.
Not "seamless", but if you set up a VPS as a reverse proxy to your site with NGINX and Certbot, you can get Let's Encrypt running with Squarespace today. Doesn't require a huge amount of technical know-how; it was only the second SSL cert I'd ever installed. If anyone is interested in detailed instructions, shoot me an email.