new squarespace domain, existing gsuite on google domain

I have a new squarespace domain, and I want to be able to forward email to an existing google workspace that uses google domains. It looks like the email forwarding option only works for domains already within the squarespace domain management. What's the best option here? Do I need to manually finagle the MX records? 

When my current google workspace domain is automatically transferred, it's unclear what will happen to the existing records etc.

Hello!

If you have a Squarespace domain, you should be able to use email forwarding to send emails to your existing Google Workspace account email address, like you suggested.

You're right that email forwarding through Squarespace is only available for Squarespace domains, though, not for third-party domains that are managed through Squarespace.

You could also add your new domain to your existing Google Workspace account. You'd add the domain through Google and you'd also need to add the Google Workspace MX records to the domain. Here's Google's guide: Add a user alias domain or secondary domain

The main difference is that adding the domain to your Google Workspace account will let you send and receive emails using that domain. Email forwarding will only let you receive emails sent through that domain.

Since Squarespace requires that customers with existing Google Workspace accounts continue managing the account through Google, I don't think you'll see any changes with that service after the domain moves to Squarespace. The only significant change is that you'll log into Squarespace to manage the domain instead of Google Domains.