I have heard research suggesting that excessive use of screens can have detrimental effects on health and productivity, does anyone have any suggestions on how to complete a PhD but minimise computer usage?

I'm in the early stages of my PhD (Year 1), so most of my working week is spent staring at my laptop screen. Some things I've done so far are: getting a standing desk set-up where I can spend at least an hour a day standing whilst working, setting my laptop screen to the 'eye comfort' or 'night light' settings (to reduce blue light exposure), and using a LED desk lamp which simulates day light and stops electrical flickering - mine has a variety of brightness and light warmth settings. After my typical working hours, I try to stay away from screens as much as possible, or at least sit further away from them.

If you are really serious about it then only read hard copies of texts, and print out papers/chapters (or send them to Kindle - if you have the paper white one it isn't really like a screen). Also you could do much of your writing, planning, and thinking on paper - and then type it up afterwards when it is already in the final stages.

I must admit I prefer reading hard copies of papers and books rather than on the screen but I don't like wasting trees either.

Kindle feels almost as good but not quite. I personally like screen reading, but as OP says, probably not the best for eyes.

The older I get, the more strain screens have on my eyes. I get exhausted very quickly. The kindle doesn't really help. I also like to jot notes on the papers and especially staple summary notes to the back to help with writing up.
Of course all of that should be past tense now.

I have the very basic kindle, not one of the Fire or other newer ones. Those more recent ones seem more like ipad screens to me, and actually omit light.

*emit

Also just to add, you can get these non-prescription glasses that block out blue light. They make screen work easier for some people. I am not sure if they reduce damage though.

First of all find out the most of the distractions that are increasing your computer usage, according to recent study the time we spend on systems 53% goes out to distractions. Find out all the distractions like searching to a term on google irrespective of the thing that it is unrelatable to your work so work around distractions i suppose half of computer usage will be saved.

That's such a good point! I probably spent less than 10% of the time doing actual work...