Accidentally pressed “Send as SMS” — how do I revert to RCS?

Settings>Apps>Messages>Send as SMS has nothing to do with sending messages of any kind to an Android phone. The only thing that setting does is cause the phone, in the event an iMessage fails, to resend it automatically as an SMS.

Since you've tried all the likely fixes (and thank you so much for listing what you tried!), I'd contact your carrier. Suggest to them, if it's possible, to turn off RCS on your account, wait until you've turned the phone off and then on again, and then add it back. However, the fact that RCS works fine with other people would suggest to me that the problem may be on the end of that particular person.

pdubs_114 wrote:

It was totally fine until I accidentally pressed the button to start sending as SMS, which tells me it’s a me problem, not a him problem.

What it tells you is that two things occured around the came time, that's all. Correlation doesn't always imply causation. If changing that switch really did what you seem to think it does (and it doesn't), it would have affected everyone you send RCS messages to. Since you've tried everything else, what is the harm in talking to the only person with whom you're having this issue?

I just don’t understand why there isn’t a button to change the default back to RCS.

Probably because there's no button to change the default to anything else. The only option in Messages that affects RCS is turning it on or off and the sub options on that page. No other settings in the Message app affect it.

Just now, looking at Settings>Apps>Messages>RCS Messaging, I see the the option to disable End-to-End Encryption. As that is currently a beta feature, try disabling it.