Some of this seems like a "grass is always greener on the other side" problem: a chat system is always going to be complicated. There's no magic framework that solves complexity, especially if you want real time chat.
There's plenty of useful and interesting rabbit holes to explore: CRDTs, tools like Y.js, web sockets, SignalR, etc.
I'd suggest taking a look at SignalR over Azure Functions as a good option, but it isn't a simplifying option. It gets it shipped faster when I write against, but that's as much because I'm familiar with it. The grass isn't always greener on the other side, you just get used to the shade of grass on your own side.
That's all unrelated to things like better responsive design for phone. That's a different section of the stack entirely. You looked at Ionic, but there's the classics like Bootstrap, and a deep rabbit hole of responsive design options. I think CSS Grid is one of the easiest tools in today's CSS toolbox for responsive design, but I also know that learning it isn't always straightforward and sometimes you have to "unlearn" habits from previous solutions such as Bootstrap's grid and CSS Flexbox to make the best use of CSS Grid. Again, all of that is various versions of "the grass isn't always greener on the other side". What do you have time to learn? What are you interested in learning? Most of those frameworks don't just "do the work for you", complicated or not, but expect you to learn their way of doing the work.
It may not even be that you are looking for "a web stack", but an "outsource solution": can you outsource chat to a vendor that does chat well? Are you trying to build a website when a sufficiently permissioned Discord or Slack group would suffice? Maybe you just need to deploy a Discord/Slack bot instead of "a website"?
Or is it that you still want a web page but you want something embeddable in it? There are much more "fully featured" web-based chat apps that speak XMPP or Matrix you can try to find and just "plug into" your website.
Go with what you know, outsource what you can, and the grass isn't always greener on the other side. You can't avoid complexity altogether, there's no magic shortcuts to most complex tasks (and chat is complex; responsive design can be complex; deployment and security will always be complex).