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

Sure, so {{foo}} is the field itself, and the {{#foo}} {{/foo}} around it is a conditional block. So if foo isn’t filled out, nothing between {{#foo}} {{/foo}} will render.

So if you did

My name is {{foo}}

and foo wasn’t defined, you’d get “My name is” in a post

but if you use

{{#foo}}My name is {{foo}}{{/foo}}

and foo is not defined, “My name is” will also be excluded from the post