Matrix exists and really isn't too bad to self-host if you just want a small number of people. (If you federate with other servers, then you have more things to worry about -- increased attack surface, more visibility leading to more potential attackers, and the risk of unintentionally storing illegal content (e.g. CSAM) sent by people from other servers.)

The UI of Element (the most popular Matrix client) is more or less in line with any other chat app, but I guess it depends what you mean by "on par to whatsapp". Biggest downside I've found is that you can't search your messages on the mobile clients.