IMHO there are some times where deep knowledge and time constraints that call for outside help. Yes, I am sure you can learn to be a good web programmer and designer, but it will require time. When developing a new web site, you need to know good coding practices in a chosen server based scripting language, javascript, html5, CSS, graphics design, maybe photography, a chosen database, SQL, database design, and secure coding practices. Almost each facet has it's own tools, and you need to master those. Web development is what I describe as "a package tied together with strings". HTML and CSS are standards, but there are multiple server based programming languages you can choose. Javascript is a standard for interactive browser pages, SQL is the database programming language but each database has variations from the base. Secure coding is dependent on the server based programming language you choose and the database. In the very beginning I thought being a programmer would enable me to do everything with computers, but as I learned and progressed, I realized that there is simply too much for 1 single person to master. .
As for your question about squarespace, understand that when you buy in to a packaged product like that, you will be tied to them. Migrating to another web hosting service will be impossible. You have to look into the future somewhat to determine if it will still fit you in a few years time. And be prepared to start from scratch if you want to migrate. The templates, databases, e-commerce and analytics are integrated, and should you need more than what each individual piece can provide, you can't do anything except submit a request for a new feature. And whether they listen or not will depend on how popular they think this feature will be before they choose to implement it. It also depends on how agile their development team is, so you will need to understand their method of operation. You will be choosing a business partner. Most small businesses can turn on a dime, so make sure they can follow your needs.