Hey everyone I was able to solve this issue! Here's how I did it:
1. From the iCloud+ panel: Add your website WITHOUT www. In my case, I had www.timrodz.dev, and changed it to timrodz.dev
2. From the iCloud prompt you get after adding your site, add the records to your Squarespace domain. Here's how mine look:
3. Wait for propagation to finish by using the `dig [website.com] [record-name]` command I wrote about in my previous comment. In my case:
dig timrodz.dev MX
dig sig1._domainkey.timrodz.dev CNAME
If you get an ANSWER SECTION in both, verify the values match your Squarespace Domain's values, and you should be set.
4. Check again inside your iCloud setup, and hopefully you're sorted! From this point, if all is good, all you have to do is add the email domains on the iCloud website.
Good luck!