So, that's 2 questions.

First, it seems that Google domains does support a DDNS A record on the bare domain. I have not tested this, but seems like you can give it a try....

Second, Instead of setting up multiple ddns configs in ddclient (since you'll just be providing the same IP address to multiple records), why not just create a CNAME to the DNS record that you are updating in ddclient? For example:

- primary.foo.com is an A record updated by ddclient

- seconary.foo.com is CNAME to promary.foo.com