RCS/iMessage conflict with iPhone and iPad after iOS 18 update

After installing iOS 18, I have been using RCS with my partner, who has an Android phone. However, he also has an iPad that I occasionally iMessage with.

Over the last few days, I've experienced the following and I'm not sure how to reconcile these issues:

• My iPhone was defaulting to iMessage his email vs texting his phone number and I couldn't get it to switch
• When creating a new message and looking up his name, his phone number was blue rather than green, as if his phone was an iPhone
• RCS/text messages not sending because it thinks I'm trying to send them to his email
• I tried removing his email address from his contact card so that it just had his phone number, but then in my messages app, it was still sort of behaving like they were the same person (contact photo transferred over to the email, even when I set a different contact photo for the email contact)
• When I put the email back into his main contact card, now when he texts me, the notification on my Apple Watch makes it look like I'm in a group message with his phone number and his email (two contact photos show up in the bubble on the watch rather than just one photo). I've checked and I don't have any duplicate contacts that I could merge
• When I delete his email contact card entirely, in my messages app, it favours the email as if the phone number doesn't exist (the message thread with his email shows up, and when I open it, it shows our entire history, including messages with his phone and email, but then there isn't a separate thread with his phone number only)
• Then when I re-add his email address to his contact card, no messages will deliver

Basically I've been going round and round trying things out, but it just causes a cascade of other issues. The only way I've been able to get texting to work is:

• delete the email from his contact card
• turn off RCS
• restart my phone
• add his email back
• turn on RCS

and then the only 'issue' remaining is where the notification on my Apple Watch appears as though it's a group chat with two contact photos (which isn't terrible, at least we're still able to communicate - it just indicates to me that there's something somewhere that still isn't clicking properly).

[Re-Titled by Moderator]