After user update automation: can it trigger from one required field, while optionally including other profile/custom fields if present?

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You should be able to trigger on your custom field and add the profile fields to the post content section.

So say the trigger is confirming the guidelines, then in the post content you could do:

Hello! I'm new here
{{#pronouns}}My pronouns are {{pronouns}}{{/pronouns}}
{{#languages}}I can speak {{languages}}{{/languages}}

Is that what you’re looking for?