I don’t suppose you’ve tried the “DJ” feature? It’s usually pretty good for me, but of course your mileage may vary.
I’ve run into the same problem of the shuffle (and “smart” shuffle) just plays the exact same thing all the time, even in a playlist with thousands of songs. Tried out YouTube music for a while, and it’s not much better. So I’ll be interested in what others say here.
The best one for me a long time ago was Pandora - for some reason I was under the impression they weren’t around anymore, but it looks like they are. But, I don’t know if they are still as great as they were back then.