Thank you for reaching out. I can certainly understand your concern with Bixby being the only option for a default assistant after the update on your Galaxy buds pro. I would first recommend to make sure Bixby is not set as your default app on your phone. Go to settings > apps > tap on Bixby> then tap on set as default and clear the default.
Next, I would give the steps in this link a try, even though you aren't setting up your assistant for SmartThings, this link will walk you through the process of selecting the proper default assistant for your buds pro: https://www.samsung.com/us/support/answer/ANS00083615/
Thank you for that information. I would recommend force stopping and clearing the cache on your Galaxy wearables app. Clearing the cache simply clears temporary files. It won't erase login credentials, downloaded files, or custom settings.
Next step, would be to uninstall and reinstall the Galaxy wearables app. This will allow for a fresh copy with the latest software available for your buds to be installed on your device.