iOS 17 beta 4 lets you change notification sounds and haptics. I'm tring this out tonight after I upgrade to 17.2 through the Apple Software Beta program. Was about to buy a new Android phone after experiencing this issue a couple days ago. Apple may have saved the day. https://appleinsider.com/articles/23/11/28/default-no-more-ios-17-beta-4-lets-you-change-notification-sounds-and-haptics

This is a show stopper for me, as I use the default notification for security and other brokerage notifications. If a way is not provided to change the default notification, I will seriously consider going back to an android phone after being on an iphone for over 10 years before. Android offers more customization features and does not FORCE users to use a specific notification sound. I will continue to search for a way to resolve this issue, and hope someone finds a resolution soon.