The problem here is not XMPP or Conversations but the closed nature of iOS which keeps apps like Conversations from being ported there. Apple does not like competition to iMessage or to its app store revenues so it fights tooth and nail to keep its precious as is now clearly on view in Europe with their ridiculous 'core technology fees' and other shenanigans.

Maybe you can give your father a non-iOS phone if that is what is keeping your experiment from succeeding? We're all on Android here, anything from stock Samsung like my mother uses to self-built LineageOS like I use and we have no problems like you describe. I video-chat daily with my mother without problems, we're using Jitsi Meet (hosted on the same server) for larger video meetings, we've used Nextcloud Talk (also hosted on that server-under-the-stairs) as well but now mostly use Conversations. Telegram also works well for video chat but that is neither self-hosted nor end-to-end encrypted so it is not a real comparison to Matrix or XMPP with OMEMO.