Sure, it's not ideal... but does it really matter? Just register your domain with this hypothetical service while you're still preparing the content for it, so

Sure, it's not ideal... but does it really matter? Just register your domain with this hypothetical service while you're still preparing the content for it, so there's time for the cert to go through before you need it. Or they could let you provide your own cert if you're in that much of a hurry. We're talking about edge-cases within edge-cases at this point: a service that (1) offers HTTPS subdomains (2) hasn't been able to get a rate-limit increase from Let's Encrypt (3) is unwilling to buy a wildcard cert (4) either is growing by hundreds of subdomains a day, or has a significant amount of clients who can't wait a few hours for a cert but won't buy their own. That's so oddly specific, I'm not surprised people aren't going out of their way to cater to it.