If you have bought a custom domain just to use with Google Apps and then created and email that you've been using for 13 years changing your email address is just not an option. So, you basically have to pay either Google or another provider.

What makes no sense to me is why Google isn't offering former legacy users the option to to migrate to the newly announced Workspace Essentials Starter while making it available to anyone with a custom email right now.

It is not even clear if you move all of your data out of Google and then delete your Google account entirely it would be allowed for you to create an Essentials Starter account with the exact same domain you just deleted.