Unchecking “Automatically collapse tab titles into icons” turns the tab bar into a sort of carousel when you have too many of them. I certainly don’t prefer it, but it’s an option if that’s your sort of thing.
While Safari 15 takes a bit to get used to (I’m still getting over the learning curve myself), Tab Groups is a great feature to share between my iPhone and computer. Safari 15 also comes with the usual host of security updates, though none seem too severe. After you update, be sure to let us know what you think of the new design — is it a breath of fresh air or just another refresh that you’ll grudgingly get used to?