You’re quite correct. The usefulness of the low-end-storage model of a ~seven old iPhone 6s model will limited on newer iOS releases.
You’re probably not going to be able to free up all that much storage, too—you will never not be manually managing an iPhone with storage insufficient for your current needs.
iPhone 6s does works decently well on iOS 15, given adequate available storage. That based on one that’s still used for app testing.
This isn’t a particularly unique situation though, as some of the bottom-end Mac configurations are similarly performance-problematic on newer macOS releases. There have been other similar cases of the capacities of low-end configurations being exceeded.
Beyond our own individual and usually-increasing storage requirements, the storage requirements for apps and operating systems all tend to increase over time.
Newer iPhones have been getting more capable, too. Any recent iPhone will substantially outperform an iPhone 6s; one with adequate available storage.