1.) Mistake #1: Plugging in the Content at the end
The content on your and client's site is the most important thing. It's why people visit websites 99% of times. That content might be text, data, videos or photos but it's all classed as content. It's very important.
Try to gather the content from your client as early in the design process as possible; this makes for much more successful projects. That's if the client is adamant, they can write the content themselves. There is an opportunity here to create another service and write the content yourself. Some designers can do this, but it's quite an undertaking. A better option would be to find a third party or freelancer that could help you.
2.) Mistake #2: Don't jump into Squarespace and start designing
Some Squarespace designers like to go as quickly as they can into Squarespace. This is a mistake; there are 2 pieces of the design process missing here.
The first is mood or style boards. These can save a lot of time and hassle down the line. It's so tempting to dive right into Squarespace, but if you do and come up with a design that the client doesn't like, then, that can be a lot of frustration and time thrown away. Mood boards are the step before design. With the brief in mind, gather some inspiration and website screenshots and graphics and then have a conversation with the client. Is this the vibe and look that they desire? Try to point them in the right direction and take the feedback on board. This may sound like such a fleeting part of the process but it's an important part of designing websites. All the best projects that have I been involved with have had the most research and mood board work done in them.
PhotoShop, Sketch or Figma
Rather than jumping into Squarespace immediately after mood boards, there is another part that some miss. Designing the site in a software application like PhotoShop to Figma, in my experience, just makes for so much more creative and successful projects.
Laying out elements is a lot quicker in a graphics package, rather than doing it in Squarespace with trial and error. There are just so many more things you can do easier in PhotoShop or Figma. With you doing things quicker, this leads to more iterations, and more iterations almost always lead to better design. Sure, there is a learning curve to a program like PhotoShop or Figma, but they are beneficial to use; the effort is worth it.