My 2 cents. I am 60 years old and have aging eyes. I am a recreational active diver (25 to 60 dives per year).
I saw just now you had a prescription mask. If you still want a large screen, then a big +1 for the Galileo Sol or Luna for a wrist mount, and the Oceanic Pro Plus for a hose mount. Personally I dive the Sol (I like wrist mount) and it is easily readable even without my prescription mask.
I find color screens to be smaller in size, and so their fonts are also smaller and harder to read in sharp focus even with the advantage of brightness and contrast. One of the best is the new Galileo 2 in 'light" mode, where it only shows depth, time, tank pressure, and time remaining. Easy to understand at a glance even with the smaller format. Not sure I could read even that without the prescription, however, where I can with the "older" Galileos (which, by the way, are perfectly fine and up to date computers).
The Shearwater is not quite as good as far as readability. Even in "rec" mode it displays more information than needed and so the fonts shrink accordingly and it is not so understandable "at a glance" as the G2. Definitely need a prescription mask for it, as it seems you have found out. As an aside, the Shearwater AI is not quite up to Scubapro or Oceanic standards, as it does not account for your safety stop in the gas time remaining calculation.
I have no experience with the Mares Icon, but the battery life seems quite poor and they are not widely used here in S. Fla. even thought they are widely available.
This is just my experience comparing readabiliity. I won't get into discussions about the "best" algorithm, user changeable versus rechargeable batteries, etc. All of these can be adapted to once you have the computer. As for algorithms, all are safe, and whether you want a more or less conservative one is up to you.