Make an appointment at your local Apple store -or send it in if you have to- and get the battery changed. End of.
I just had the battery changed on my 6S+ which was showing 81% and which would go about 1.5 hours between charges and it now can go at least a whole day. No issues with anything on latest OS (but then I didn't anyway).
It will cost you under $50 depending where you are.
Lithium ion batteries work well until they are nearly drained right up to the point where they are at the end of their life and then performance effectively falls off a cliff. It isn't linear and it's just gets a little bit worse: they are awful
There really is nothing else to be said about it. A 6S with a healthy battery should work perfectly on the latest OS so if it doesn't then you have other issues, and that battery is dead. (altogether now- it has ceased to be, joined the choir invisible etc. )
Oh maybe I could add that the recommendation is to change them when they go under 85% and in the battery health app where it gives you the percentage, you should have a message just under the figure telling you that.