What's the most secure messaging app that normal people will actually use?

Signal is still the gold standard for security, full stop, and it is free and easy once installed. The install friction you mention is real though. Honest take on WhatsApp: the message contents are genuinely end to end encrypted using the Signal protocol, so the text of your chats is protected. The trade-off is metadata (who you talk to, when, how often) which Meta can see and does use. So it is not a false sense of security on the content, but it is not private in the fuller sense either. For a family where adoption matters most, WhatsApp with disappearing messages turned on is a reasonable floor, and Signal for the people who will install it. Telegram, worth flagging, is not end to end by default (only Secret Chats are), so do not treat regular Telegram as private.

Signal. Everything else is a compromise. The install is one screen, most people manage it if you send them the link directly.

Do not sleep on the metadata point above. Encrypted content with a fully visible social graph is a very different privacy promise than people assume. If that matters to you, Signal collects almost none of it, which is the real reason to prefer it.