Ah, shucks. I never managed to find a conclusive "acknowledgment" of my issue, so once Safari 14 rolled around, I only checked whether I can make my issue reoccur and shrugged it off as resolved. Guess I was wrong, because your description feels eerily familiar. Maybe we're using it too fast.
Agreed about the user experience. Really unpleasant to work around some indiscernible and apparently unfixable problems. I feel like in a "death by a thousand papercuts" scenario with Apple software, but I suppose that is a topic unto itself.
With Firefox I sometimes notice oddities with how fonts are rendered, and it generally feels "less native", but at least I don't run into miscellaneous issues like that anymore.
I've experienced something similar before Safari 14: when opening Safari for the first time and quickly pasting the URL to the address bar and hitting Enter, the URL would just disappear and I'd hear a bell sound. Had to wait a second to paste the URL and have it actually go to that page. These little things contribute to a general feeling that the browser is not polished enough for everyday use.