Unfortunately the 6s is now a retired device. No device today can run a modern operating system with that low of a base hard drive. There truly isn't an option besides upgrading to a newer device with more memory.

The phone will work as a phone and stock apps will work for the foreseeable future. Added apps and anything requiring updates will cease to function at some point.

I use iPhone 6s 64 GB variant. Here's how I manager my storage.

1. For photos I usually back them up to my computer using iTunes or by directly connecting it to my computer to backup the DCIM folder. Then I remove any photos I do not want to keep on the iPhone to make some room to take more pictures or download media from apps. I try to stay away from iCloud for backing up photos. Self storage of your photos is good in a way. If you feel otherwise, try using iCloud as a solution.
2. Offload unused apps - Your app data is preserved on your phone but the apps are in a way removed to recover storage space.
3. Go to settings > general > iPhone storage and see which apps are using up more storage space. Deleting an app removes the app + data. Offloading will remove just the unused app but retain the data.
4. Clear safari's browser cache/cookies or browsing data regularly since you have just a 16GB capacity.
5. Delete old mail storage and iMessage / messages. You can setup messages to keep data for a particular period of time and auto delete.
6. Also see if this helps to clear up some 'system data' - https://www.reddit.com/r/ios/comments/u8jo49/comment/i5ll7sa/?context=3 . iOS usually handles this automatically but this is a workaround as a manual approach.

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It’s a retired device bro there are many cheap iPhones out there right now just upgrade it

No shaming but 6s is worth less than dirt of equal weight. Let it go man. My friend with 14 says the same to me haha. I have a xs max though