You're a victim of someone who read something like the "Associated Press Stylebook" and mistook it for a universal authority. It's not.
The issue you bring up is a style issue -- NOT a matter of right and wrong. And writers aren't really responsible for knowing style.
Yes, most publications/publishers eschew numerals at the start of sentences (with some exceptions). The "Chicago Manual of Style," which most book publishers follows, agrees. It says to spell out numbers at beginnings of sentences -- even years: "It happened in 1776," but, "Twenty twenty-one should be an interesting year." AP disagrees on that last point. Per AP: "2021 should be an interesting year."
If I were you, I would take a very broad and oversimplied approach to the "Chicago Manual of Style." (Bear in mind that Chicago has a WHOLE CHAPTER full of rules and exceptions to the rules and exceptions to the exceptions -- just on when to write numerals or spell out numbers. Nobody expects writers to know them all.)
So, I would:
* Spell out most numbers (except really odd ones like 1,188) -- that's a main premise of Chicago.
* Recast sentences to avoid numbers at the beginning but, if that doesn't work so well, I would simply write it however I wanted and let the copy editors worry about it.
That's their job. Not yours. Yes, you want to look like a seasoned pro, but nobody expects writers to commit to memory literally dozens of specific rules and instructions and exceptions.
* In the future, take every bit of grammar/punctuation/style advice with a grain of salt. Style books disagree. They disagree on style matters, which exist purely for consistency's sake. And they also agree on grammar issues, because grammar matters are disputed, too.
P.S. DON'T listen to the Elements of Style. That was a style guide for one college professor's classroom a hundred years ago. Some of its wisdom still applies today. But it's not an official style guide that publishers follow today.