For what it’s worth, I’ve kept rails-ujs to submit forms for now. I’m using it like this:
Rails.fire(form, "submit")
Sure enough, for a form (without a submit button) that’s in a Turbo Frame this does trigger turbo requests.
Now there has been some discussion about keeping rails-ujs, but I’m not sure if this particular function should be on the list (@dhh?). And I’m not even sure if I’m abusing stuff or if Rails.fire is indeed a sure-fire way of getting this to work.