I use Boost and I use quizzes.
If I want to avoid the student forgetting to submit, when I set up a timed quiz, I set "When time expires" to "Open attempts are submitted automatically." Also, this feature (I think) will close any quizzes after the due date.
In Boost, "Finish Attempt" is colored. But you are correct, that "Submit all and finish" is grey, giving it visual importance equal to "Return to attempt." Hmmm, I don't know. I guess that you could submit a feature request. Or you might be able to add some custom CSS and change its color.
In practice, I begin my course with a simple student survey. I stress that this survey also shows that students need to "Submit all and finish." So I tend to train my students to click that last button. Again, not a significant problem because open quizzes will be closed automatically by Moodle.
And again, in practice and having been using Moodle for 10 years, I have never had a "disaster" like the one that you describe.