Hi there! I have a bit of a problem with two Workspace accounts (outlined below).
Unfortunately the best way to have gone about this was to simply add the new domain as another domain to your existing Google Workspace, but you've missed that opportunity now.
There isn't a good way to do this. you can use Google Workspace's built-in tools to migrate email and export/import contacts and calendar. Drive, however, is not that straight forward at all. If you don't mind all the URLs changing then there are third-party tools that will do everything for you but they make copies of the documents/files and all the URLs change (so if you find an email with a link to doc that link will not work).
If you're happy to put a lot of effort in you can manually create a Shared Drive in the new Workspace and add your old account as a manager to the shared drive and manually migrate your content from the old Workspace account to the new Workspace Shared Drive, but you cannot migrate folders, only files, so you would need to recreate your folder structure (and your folder links will change) and move the files folder-by-folder.
This type of Google to Google migrations is pretty common and can be done but there are a lot of caveats. One of the main challenges the data doesn't actually move, the data is copied into the new environment. So links change for docs and sites. Documents have to be reshared. Calendar events are recreated. The old files and calendar events etc still exist as well so it can become confusing to users external. Typically the old environment will get shutdown once the move has been made to help.
It sounds like this may be just for yourself and not for a large user set. If this is for a large group of users I would highly recommend using a partner to help instead of doing it on your own. If it's just you you may end up doing a lot of things manually. A migration tool is typically used for larger migrations but could be used for any size even 1 user. One popular tool is Cloudm. This link is their watchpoints for this type of migration. Even if you don't use their tool this list would still tell you what can and can't be migrated.
Both @newman and @StephenHind answers are right.
How you do it is going to be based on how many users, what information you need to transfert (only mail, everything..) and how much time you are willing to commit to the task.
Also, the way you bought Google Workspace. There may be cerain constraints on the updating of your primary domain, the releasing of that doamin etc due to the fact that you went trhough Squarespace and not directly or through a Google Services and Sales Partner (https://cloud.google.com/find-a-partner/)
You may need to check that first.
In any case, there are a number of partner available to help you on this. and it is possible to DIY it. but needs some time and effort to get it right 🙂