I think the issue is that Delta Chat doesn’t know that it’s a Google Workspace email account. Would it be possible to query the MX records? The MX records for a Google Workspace account are a dead giveaway:
$ dig mx example.com
example.com. 300 IN MX 5 alt1.aspmx.l.google.com.
example.com. 300 IN MX 5 alt2.aspmx.l.google.com.
example.com. 300 IN MX 10 alt3.aspmx.l.google.com.
example.com. 300 IN MX 10 alt4.aspmx.l.google.com.
example.com. 300 IN MX 1 aspmx.l.google.com.
Delta Chat version
Delta Chat iOS 1.24.5 (67)
Expected behavior
Use email and password to log in to Delta Chat.
Actual behavior
“Log In to Your Server” popup appears, but stops at 60%.
Steps to reproduce the problem
Assuming that this is an issue with any Google Workspace email with a custom domain:
Actually Oauth2 is supported by the core and MX record is queried, but as far as I know the corresponding UI flow for initial setup exists only on Android. You can even create an account using Oauth2, export a backup and import it on iOS or desktop, and it will work.
Looks like there is an issue already: add oauth2 support · Issue #92 · deltachat/deltachat-ios · GitHub
The issue was created when core did not support OAuth2 at all, then even Google Workspace (GSuite) support was added but the related code in iOS is still disabled.