I too used Squarespace simply because my website was hosted there and came to the same conclusion. The power of having everything connected was great, but the benefits of convenience were outweighed by the rigid and costly features.
I've been using Substack for a few months and much prefer this for my mailing list. Plus, if I ever choose to use a paid newsletter for bonus features, it's a great resource. The statistics features are a little sub-par, but overall this has been my best mailing service experience.
Thanks for the article!