It's definitely possible to set up your Sonic DNS to work with Squarespace and make all the changes they require to get things working.
Looking at https://support.squarespace.com/hc/en-u ... space-site
, it looks like they want 4 "A" records set up to point your domain to several of their IP addresses.
In addition to the A records, "www.yourdomain.com" should be a CNAME record of "ext-cust.squarespace.com" (you may need to delete an existing record for "www.yourdomain.com" which points to yourdomain.com).
They may also have you set a verification record, which is a random string, that is a CNAME record pointing to verify.squarespace.com.
Any existing A records pointing to Sonic's hosting, for example 126.96.36.199 (parked.sonic.net) should be deleted.
If you're still stuck and can provide more specifics, or private message me your domain name and/or Sonic username, I can take a closer look.