So I got a reply back:
Thank you for contacting Google Domains.
I'm afraid that Google Domains does not yet support API that allows you to automate or modify existing dns records on the domain's settings. However, if you're referring on adding TXT records from ACME v2, you may follow the steps below:
Login to Google Domains page.
Click DNS tab.
Scroll down to Custom resource records.
Add these records for TXT records.
Name: @
Type: Select TXT
TTL: 1h
Data: Txt records from ACME v2.Click Add.
Please note that for DNS changes to take effect, it may take a propagation period of several minutes or up to 24-48 hours across the entire internet.
For further questions, feel free to reply to this email or visit the Google Domains contact us page.
I suppose that completely nerfs the possibility of having wildcard certs without manually entering the TXT records. I'm switching away from Google Domains, and I would recommend anyone else who wants automated DNS challenges or wildcard certificates to do the same.
That said, one can set up their own DNS server and then use that to do the challenges, but it's something that I believe the DNS provider should handle with an API, honestly. Much less work for the end user.