martin What I need to do is test that with print. Almost everything is digital these days. We spend hours staring at our phones, completely mesmerized with what we are seeing or reading, and forgetting to look away and refocus our eyes from time to time. Holding any clenched muscle for a prolonged period of time would be difficult.
I need to read some book or instruction manual for multiple hours at a time, resisting any temptation to use my phone during that stretch of time, to see if the same familiar ache starts to build up. If it does not, then it's possible there is still something with the digital display of the iPhone 6S+ that makes the issue worse.
I run my iPhone with pretty low brightness and with NightShift mode on at all times. Since holding a phone is half the focal distance of looking at a computer monitor a full arm's length distance away, I only use my phone for minutes at a time when I need it. If I find an article that I like, I will sometimes email it to myself so I can read it on my larger and further away desktop monitor so I don't strain my eyes as fast.
Little strategies like this have seemed to make a difference.