OK, so, I said I'd download Dia and give it a try. I did.
It's based on Chromium (which surprised me), not Apple's WebKit, so it has a few advantages and disadvantages.
The advantages are that a few websites seem to just work better on Chromium-based browsers (since Chrome itself is so prevalent). Ones I find work a bit better are Reddit and, believe it or not, MacRumors. Both just seem more responsive. Reddit really bogs down on Safari, at least for me. Some of the internal work webpages I need to log into for work also seem to have better formatting on Chromium-based browsers than they do on Safari, but these differences are minor. The disadvantage is that, in general, Safari otherwise is the faster browser for many other sites, though my computer is fast enough that it probably makes no real-world difference.
A disadvantage is that since it's Chromium-based, some of the non-native UI elements are out of place, especially on Tahoe. The Dia developers have improved things a little by using native drop-downs on the favorites bar versus what ever internal routines are used on Chrome and Edge, but it still feels a bit "off" as compared with a completely native UI, especially, again, on Tahoe.
As for the AI aspect of the browser, I've done a few things with it and it seems handy enough when there's a proper use-case for it (like asking questions and summarizing things on a particular page like telling me how many times someone has been assigned someplace on a work schedule QGenda spreadsheet -- which it got wrong multiple times, by the way), but I'm not sure that, at least in my workflow, it's enough to get me to change over from Safari. But I'll keep trying to find the killer use for it and report back if I do.