Can CB send confirmation email, but no "welcome" email? Asking for a friend. :-)

You wouldn't be conditioning against what the email will become, but what you're trying to replace. So if you're trying to replace the confirmation email you're replace var4 (subject) and var5 (body) with whatever you want and you'd condition against var4 or var5 to make sure it's a confirmation email.

So with the default pending email subject of YOUR_REGISTRATION_IS_PENDING_APPROVAL_SUBJECT which gets translated to "Your sign up request is pending approval", which will be sent to that trigger as translated. You could then for example use the below condition.

Value: Custom > Value
Custom Value: [var4]
Operator: Does Contain
Value: Your sign up request is pending approval

You could even set the pending email subject to an untranslated key and just check against that. So for example lets set it to CONFIRMATION_EMAIL_SUBJECT. We can then condition it as follows.

Value: Custom > Value
Custom Value: [var4]
Operator: Does Contain
Value: CONFIRMATION_EMAIL_SUBJECT

That in theory should work, but have never actually tried doing this before. Your custom PHP code would then alter the email however you like.