Element/Matrix is pretty much what you’re looking for if you need self hosted. Could also look into Jabber/XMPP or IRC but Matrix is going to be way more modern and useable.

It feels really weird to go to one website and enter my credentials for another website. How secure is that? I guess whatever app I’m using could be storing credentials instead of using an API, but the fact I can see a URL and enter the wrong creds from my password manager feels off.

I have 15-20 domains with Google so it’ll be a pain to manage something else. Even if I can’t trust Google’s privacy or product continuity, I do trust Google’s security and have multi factor enabled already, so having my domains protected by them is a great thing. I’m pretty much always logged into my Google account on my PC so it’s one fewer account to manage.

Squarespace is probably fine, but I bought my first domains through a web builder and web host years ago and they slowly raised prices and reduced features until I had to leave them and moved to Google.